SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Schedule 13G (Rule 13d-102) INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT TO RULES 13d-1 (b), (c) AND (d) AND AMENDMENTS THERETO FILED PURSUANT TO 13d-2 (b) (Amendment No. 1)* Momenta Pharmaceuticals, Inc. (Name of Issuer) Common Stock (Title of Class of Securities) 60877T101 (CUSIP Number) December 30, 2005 (Date of Event Which Requires Filing of this Statement) Check the appropriate box to designate the rule pursuant to which this Schedule is filed: [ ] Rule 13d-1(b) [ ] Rule 13d-1(c) [X] Rule 13d-1(d) * The remainder of this cover page shall be filled out for a reporting person's initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page. The information required on the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes). (Continued on following pages) Page 1 of 16 Exhibit Index on Page 13 CUSIP NO. 60877T101 13 G Page 2 of 16 -------------------------------------------------------------------------------- 1 NAME OF REPORTING SS OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Polaris Venture Partners III, L.P. ("PVP III") -------------------------------------------------------------------------------- 2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP* (a) [ ] (b) [X] -------------------------------------------------------------------------------- 3 SEC USE ONLY -------------------------------------------------------------------------------- 4 CITIZENSHIP OR PLACE OF ORGANIZATION Delaware -------------------------------------------------------------------------------- 5 SOLE VOTING POWER 1,743,132 shares, except that Polaris Venture Management Co. III, L.L.C. ("Polaris III"), the general partner of PVP III, may be deemed to have sole power to vote these shares, and Jonathan A. Flint ("Flint"), Terrance G. McGuire ("McGuire") and Alan G. Spoon ("Spoon"), the managing members of Polaris III, may be deemed to have shared power to vote these shares. NUMBER OF ---------------------------------------------------------------- SHARES 6 SHARED VOTING POWER BENEFICIALLY See response to row 5. OWNED BY EACH REPORTING ---------------------------------------------------------------- PERSON 7 SOLE DISPOSITIVE POWER WITH 1,743,132 shares, except that Polaris III, the general partner of PVP III, may be deemed to have sole power to dispose of these shares, and Flint, McGuire and Spoon, the managing members of Polaris III, may be deemed to have shared power to dispose of these shares. ---------------------------------------------------------------- 8 SHARED DISPOSITIVE POWER See response to row 7. -------------------------------------------------------------------------------- 9 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,743,132 -------------------------------------------------------------------------------- 10 CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES -------------------------------------------------------------------------------- 11 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 5.72% -------------------------------------------------------------------------------- 12 TYPE OF REPORTING PERSON PN -------------------------------------------------------------------------------- CUSIP NO. 60877T101 13 G Page 3 of 16 -------------------------------------------------------------------------------- 1 NAME OF REPORTING SS OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Polaris Venture Partners Entrepreneurs' Fund III, L.P. ("Entrepreneurs' III") -------------------------------------------------------------------------------- 2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP* (a) [ ] (b) [X] -------------------------------------------------------------------------------- 3 SEC USE ONLY -------------------------------------------------------------------------------- 4 CITIZENSHIP OR PLACE OF ORGANIZATION Delaware -------------------------------------------------------------------------------- 5 SOLE VOTING POWER 45,301 shares, except that Polaris III, the general partner of Entrepreneurs' III, may be deemed to have sole power to vote these shares, and Flint, McGuire and Spoon, the managing members of Polaris III, may be deemed to have shared power to vote these shares. ---------------------------------------------------------------- NUMBER OF 6 SHARED VOTING POWER SHARES See response to row 5. BENEFICIALLY OWNED BY EACH ---------------------------------------------------------------- REPORTING 7 SOLE DISPOSITIVE POWER PERSON 45,301 shares, except that Polaris III, the general partner WITH of Entrepreneurs' III, may be deemed to have sole power to dispose of these shares, and Flint, McGuire and Spoon, the managing members of Polaris III, may be deemed to have shared power to dispose of these shares. ---------------------------------------------------------------- 8 SHARED DISPOSITIVE POWER See response to row 7. -------------------------------------------------------------------------------- 9 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 45,301 -------------------------------------------------------------------------------- 10 CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES -------------------------------------------------------------------------------- 11 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 0.14% -------------------------------------------------------------------------------- 12 TYPE OF REPORTING PERSON PN -------------------------------------------------------------------------------- CUSIP NO. 60877T101 13 G Page 4 of 16 -------------------------------------------------------------------------------- 1 NAME OF REPORTING SS OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Polaris Venture Partners Founders' Fund III, L.P. ("Founders' III") -------------------------------------------------------------------------------- 2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP* (a) [ ] (b) [X] -------------------------------------------------------------------------------- 3 SEC USE ONLY -------------------------------------------------------------------------------- 4 CITIZENSHIP OR PLACE OF ORGANIZATION Delaware -------------------------------------------------------------------------------- 5 SOLE VOTING POWER 27,497 shares, except that Polaris III, the general partner of Founders' III, may be deemed to have sole power to vote these shares, and Flint, McGuire and Spoon, the managing members of Polaris III, may be deemed to have shared power to vote these shares. ---------------------------------------------------------------- NUMBER OF 6 SHARED VOTING POWER SHARES See response to row 5. BENEFICIALLY OWNED BY EACH ---------------------------------------------------------------- REPORTING 7 SOLE DISPOSITIVE POWER PERSON 27,497 shares, except that Polaris III, the general partner WITH of Founders' III, may be deemed to have sole power to dispose of these shares, and Flint, McGuire and Spoon, the managing members of Polaris III, may be deemed to have shared power to dispose of these shares. ---------------------------------------------------------------- 8 SHARED DISPOSITIVE POWER See response to row 7. -------------------------------------------------------------------------------- 9 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 27,497 -------------------------------------------------------------------------------- 10 CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES -------------------------------------------------------------------------------- 11 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 0.09% -------------------------------------------------------------------------------- 12 TYPE OF REPORTING PERSON PN -------------------------------------------------------------------------------- CUSIP NO. 60877T101 13 G Page 5 of 16 -------------------------------------------------------------------------------- 1 NAME OF REPORTING SS OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Polaris Venture Management Co. III, L.L.C. ("Polaris III") -------------------------------------------------------------------------------- 2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP* (a) [ ] (b) [X] -------------------------------------------------------------------------------- 3 SEC USE ONLY -------------------------------------------------------------------------------- 4 CITIZENSHIP OR PLACE OF ORGANIZATION Delaware -------------------------------------------------------------------------------- 5 SOLE VOTING POWER 1,816,380 shares, of which 1,743,132 are directly owned by PVP III, 45,301 are directly owned by Entrepreneurs' III and 27,497 are directly owned by Founders' III. Polaris III, the general partner of PVP III, Entrepreneurs' III and Founders' III, may be deemed to have sole power to vote these shares, and Flint, McGuire and Spoon, the managing members of Polaris III, may be deemed to have shared power to vote these shares. ---------------------------------------------------------------- NUMBER OF 6 SHARED VOTING POWER SHARES See response to row 5. BENEFICIALLY OWNED BY EACH ---------------------------------------------------------------- REPORTING 7 SOLE DISPOSITIVE POWER PERSON 1,816,380 shares, of which 1,743,132 are directly owned by WITH PVP III, 45,301 are directly owned by Entrepreneurs' III and 27,497 are directly owned by Founders' III. Polaris III, the general partner of PVP III, Entrepreneurs' III and Founders' III, may be deemed to have sole power to dispose of these shares, and Flint, McGuire and Spoon, the managing members of Polaris III, may be deemed to have shared power to dispose of these shares. ---------------------------------------------------------------- 8 SHARED DISPOSITIVE POWER See response to row 7. -------------------------------------------------------------------------------- 9 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,816,380 -------------------------------------------------------------------------------- 10 CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES -------------------------------------------------------------------------------- 11 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 5.96% -------------------------------------------------------------------------------- 12 TYPE OF REPORTING PERSON OO -------------------------------------------------------------------------------- CUSIP NO. 60877T101 13 G Page 6 of 16 -------------------------------------------------------------------------------- 1 NAME OF REPORTING SS OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Jonathan A. Flint ("Flint") -------------------------------------------------------------------------------- 2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP* (a) [ ] (b) [X] -------------------------------------------------------------------------------- 3 SEC USE ONLY -------------------------------------------------------------------------------- 4 CITIZENSHIP OR PLACE OF ORGANIZATION U.S. Citizen -------------------------------------------------------------------------------- 5 SOLE VOTING POWER 369 shares. ---------------------------------------------------------------- 6 SHARED VOTING POWER 1,816,380 shares, of which 1,743,132 are directly owned by PVP III, 45,301 are directly owned by Entrepreneurs' III and 27,497 are directly owned by Founders' III. Polaris III is the general partner of PVP III, Entrepreneurs' III and Founders' III. Flint is a managing member of Polaris III, and may be deemed to have shared power to vote these shares. NUMBER OF SHARES ---------------------------------------------------------------- BENEFICIALLY 7 SOLE DISPOSITIVE POWER OWNED BY EACH 369 shares. REPORTING PERSON ---------------------------------------------------------------- WITH 8 SHARED DISPOSITIVE POWER 1,816,380 shares, of which 1,743,132 are directly owned by PVP III, 45,301 are directly owned by Entrepreneurs' III and 27,497 are directly owned by Founders' III. Polaris III is the general partner of PVP III, Entrepreneurs' III and Founders' III. Flint is a managing member of Polaris III, and may be deemed to have shared power to dispose of these shares. -------------------------------------------------------------------------------- 9 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,816,749 -------------------------------------------------------------------------------- 10 CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES -------------------------------------------------------------------------------- 11 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 5.96% -------------------------------------------------------------------------------- 12 TYPE OF REPORTING PERSON IN -------------------------------------------------------------------------------- CUSIP NO. 60877T101 13 G Page 7 of 16 -------------------------------------------------------------------------------- 1 NAME OF REPORTING SS OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Terrance G. McGuire ("McGuire") -------------------------------------------------------------------------------- 2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP* (a) [ ] (b) [X] -------------------------------------------------------------------------------- 3 SEC USE ONLY -------------------------------------------------------------------------------- 4 CITIZENSHIP OR PLACE OF ORGANIZATION U.S. Citizen -------------------------------------------------------------------------------- 5 SOLE VOTING POWER 0 shares. ---------------------------------------------------------------- 6 SHARED VOTING POWER 1,816,380 shares, of which 1,743,132 are directly owned by PVP III, 45,301 are directly owned by Entrepreneurs' III and 27,497 are directly owned by Founders' III. Polaris III is the general partner of PVP III, Entrepreneurs' III and NUMBER OF Founders' III. McGuire is a managing member of Polaris III, SHARES and may be deemed to have shared power to vote these BENEFICIALLY shares. OWNED BY EACH REPORTING ---------------------------------------------------------------- PERSON 7 SOLE DISPOSITIVE POWER WITH 0 shares. ---------------------------------------------------------------- 8 SHARED DISPOSITIVE POWER 1,816,380 shares, of which 1,743,132 are directly owned by PVP III, 45,301 are directly owned by Entrepreneurs' III and 27,497 are directly owned by Founders' III. Polaris III is the general partner of PVP III, Entrepreneurs' III and Founders' III. McGuire is a managing member of Polaris III, and may be deemed to have shared power to dispose of these shares. -------------------------------------------------------------------------------- 9 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,816,380 -------------------------------------------------------------------------------- 10 CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES -------------------------------------------------------------------------------- 11 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 5.96% -------------------------------------------------------------------------------- 12 TYPE OF REPORTING PERSON IN -------------------------------------------------------------------------------- CUSIP NO. 60877T101 13 G Page 8 of 16 -------------------------------------------------------------------------------- 1 NAME OF REPORTING SS OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Alan G. Spoon ("Spoon") -------------------------------------------------------------------------------- 2 CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP* (a) [ ] (b) [X] -------------------------------------------------------------------------------- 3 SEC USE ONLY -------------------------------------------------------------------------------- 4 CITIZENSHIP OR PLACE OF ORGANIZATION U.S. Citizen -------------------------------------------------------------------------------- 5 SOLE VOTING POWER 0 shares. ---------------------------------------------------------------- 6 SHARED VOTING POWER 1,816,380 shares, of which 1,743,132 are directly owned by PVP III, 45,301 are directly owned by Entrepreneurs' III and 27,497 are directly owned by Founders' III. Polaris III is the general partner of PVP III, Entrepreneurs' III and NUMBER OF Founders' III. Spoon is a managing member of Polaris III, SHARES and may be deemed to have shared power to vote these BENEFICIALLY shares. OWNED BY EACH REPORTING ---------------------------------------------------------------- PERSON 7 SOLE DISPOSITIVE POWER WITH 0 shares. ---------------------------------------------------------------- 8 SHARED DISPOSITIVE POWER 1,816,380 shares, of which 1,743,132 are directly owned by PVP III, 45,301 are directly owned by Entrepreneurs' III and 27,497 are directly owned by Founders' III. Polaris III is the general partner of PVP III, Entrepreneurs' III and Founders' III. Spoon is a managing member of Polaris III, and may be deemed to have shared power to dispose of these shares. -------------------------------------------------------------------------------- 9 AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,816,380 -------------------------------------------------------------------------------- 10 CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES -------------------------------------------------------------------------------- 11 PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 5.96% -------------------------------------------------------------------------------- 12 TYPE OF REPORTING PERSON IN -------------------------------------------------------------------------------- CUSIP NO. 60877T101 13 G Page 9 of 16 ITEM 1(A). NAME OF ISSUER Momenta Pharmaceuticals, Inc. ITEM 1(B). ADDRESS OF ISSUER'S PRINCIPAL EXECUTIVE OFFICES 43 Moulton Street Cambridge, Massachusetts 02138 ITEM 2(A). NAME OF PERSONS FILING This Statement is filed by Polaris Venture Partners III, L.P., a Delaware limited partnership ("PVP III"), Polaris Venture Partners Entrepreneurs' Fund III, L.P., a Delaware limited partnership ("Entrepreneurs' III"), Polaris Venture Partners Founders' Fund III, L.P., a Delaware limited partnership ("Founders' III"), Polaris Venture Management Co. III, L.L.C., a Delaware limited liability company ("Polaris III"), Jonathan A. Flint ("Flint"), Terrance G. McGuire ("McGuire") and Alan G. Spoon ("Spoon"). Flint, McGuire and Spoon are the managing members of Polaris III. The foregoing entities and individuals are collectively referred to as the "Reporting Persons." Polaris III, the general partner of PVP III, Entrepreneurs' III and Founders' III, may be deemed to have sole power to vote and sole power to dispose of shares of the issuer directly owned by PVP III, Entrepreneurs' III and Founders' III. Flint, McGuire and Spoon are the managing members of Polaris III and may be deemed to have shared power to vote and shared power to dispose of shares of the issuer directly owned by PVP III, Entrepreneurs' III and Founders' III. ITEM 2(B). ADDRESS OF PRINCIPAL OFFICE The address for each of the Reporting Persons is: Polaris Venture Partners 1000 Winter Street, Suite 3350 Waltham, Massachusetts 02451 ITEM 2(C) CITIZENSHIP PVP III, Entrepreneurs' III and Founders' III are Delaware limited partnerships; Polaris III is a Delaware Limited Liability Company; Flint, McGuire and Spoon are United States citizens. ITEM 2(D) AND (E). TITLE OF CLASS OF SECURITIES AND CUSIP NUMBER Common Stock CUSIP # 60877T101 ITEM 3. Not Applicable ITEM 4. OWNERSHIP The following information with respect to the ownership of the Common Stock of the issuer by the persons filing this Statement is provided as of December 30, 2005: (a) Amount beneficially owned: See Row 9 of cover page for each Reporting Person. (b) Percent of Class: CUSIP NO. 60877T101 13 G Page 10 of 16 See Row 11 of cover page for each Reporting Person. (c) Number of shares as to which such person has: (i) Sole power to vote or to direct the vote: See Row 5 of cover page for each Reporting Person. (ii) Shared power to vote or to direct the vote: See Row 6 of cover page for each Reporting Person. (iii) Sole power to dispose or to direct the disposition of: See Row 7 of cover page for each Reporting Person. (iv) Shared power to dispose or to direct the disposition of: See Row 8 of cover page for each Reporting Person. ITEM 5. OWNERSHIP OF FIVE PERCENT OR LESS OF A CLASS. Not applicable. ITEM 6. OWNERSHIP OF MORE THAN FIVE PERCENT ON BEHALF OF ANOTHER PERSON. Under certain circumstances set forth in the limited partnership agreements of PVP III, Entrepreneurs' III and Founders' III, the general and limited partners of each of such entities may be deemed to have the right to receive dividends from, or the proceeds from, the sale of shares of the issuer owned by each such entity of which they are a partner. ITEM 7. IDENTIFICATION AND CLASSIFICATION OF THE SUBSIDIARY WHICH ACQUIRED THE SECURITY BEING REPORTED ON BY THE PARENT HOLDING COMPANY Not applicable. ITEM 8. IDENTIFICATION AND CLASSIFICATION OF MEMBERS OF THE GROUP. Not applicable. ITEM 9. NOTICE OF DISSOLUTION OF GROUP. Not applicable. ITEM 10. CERTIFICATION. Not applicable. Page 11 of 16 SIGNATURES After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct. Dated: February 13, 2006 POLARIS VENTURE PARTNERS III, L.P., /s/ Kevin Littlejohn a Delaware Limited Partnership ---------------------------------------- Signature Kevin Littlejohn Authorized Signatory POLARIS VENTURE PARTNERS /s/ Kevin Littlejohn ENTREPRENEURS' FUND III, L.P., ---------------------------------------- a Delaware Limited Partnership Signature Kevin Littlejohn Authorized Signatory POLARIS VENTURE PARTNERS /s/ Kevin Littlejohn FOUNDERS' FUND III, L.P., ---------------------------------------- a Delaware Limited Partnership Signature Kevin Littlejohn Authorized Signatory POLARIS VENTURE MANAGEMENT CO. III, L.L.C., /s/ Kevin Littlejohn a Delaware Limited Liability Company ---------------------------------------- Signature Kevin Littlejohn Authorized Signatory JONATHAN A. FLINT /s/ Kevin Littlejohn ---------------------------------------- Signature Kevin Littlejohn Authorized Signatory Page 12 of 16 TERRANCE G. MCGUIRE /s/ Kevin Littlejohn ---------------------------------------- Signature Kevin Littlejohn Authorized Signatory ALAN G. SPOON /s/ Kevin Littlejohn ---------------------------------------- Signature Kevin Littlejohn Authorized Signatory Page 13 of 16 EXHIBIT INDEX Found on Sequentially Exhibit Numbered Page ------- ------------- Exhibit A: Agreement of Joint Filing 14 Exhibit B: Reference to Kevin Littlejohn as Authorized Signatory 16 Page 14 of 16 EXHIBIT A Agreement of Joint Filing The undersigned hereby agree that a single Schedule 13G (or any amendment thereto) relating to the Common Stock of Momenta Pharmaceuticals, Inc. shall be filed on behalf of each of the undersigned and that this Agreement shall be filed as an exhibit to such Schedule 13G. Dated: February 13, 2005 POLARIS VENTURE PARTNERS III, L.P., /s/ Kevin Littlejohn a Delaware Limited Partnership -------------------------------------- Signature Kevin Littlejohn Authorized Signatory POLARIS VENTURE PARTNERS /s/ Kevin Littlejohn ENTREPRENEURS' FUND III, L.P., -------------------------------------- a Delaware Limited Partnership Signature Kevin Littlejohn Authorized Signatory POLARIS VENTURE PARTNERS /s/ Kevin Littlejohn FOUNDERS' FUND III, L.P., -------------------------------------- a Delaware Limited Partnership Signature Kevin Littlejohn Authorized Signatory POLARIS VENTURE /s/ Kevin Littlejohn MANAGEMENT CO. III, L.L.C., -------------------------------------- a Delaware Limited Liability Company Signature Kevin Littlejohn Authorized Signatory JONATHAN A. FLINT /s/ Kevin Littlejohn -------------------------------------- Signature Kevin Littlejohn Authorized Signatory Page 15 of 16 TERRANCE G. MCGUIRE /s/ Kevin Littlejohn -------------------------------------- Signature Kevin Littlejohn Authorized Signatory ALAN G. SPOON /s/ Kevin Littlejohn -------------------------------------- Signature Kevin Littlejohn Authorized Signatory Page 16 of 16 EXHIBIT B REFERENCE TO KEVIN LITTLEJOHN AS AUTHORIZED SIGNATORY Kevin Littlejohn has signed the enclosed documents as Authorized Signatory. Note that copies of the applicable Agreement to File Jointly and Statement Appointing Designated Filer and Authorized Signatory are already on file with the appropriate agencies.