FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person * Stanfill John L   2. Issuer Name and Ticker or Trading Symbol COSTAR GROUP INC [CSGP] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director _____ 10% Owner __X__ Officer (give title below) _____ Other (specify below) Sr. VP-Sales, Customer Service (Last) (First) (Middle) C/O COSTAR GROUP, INC., 1331 L STREET, NW 3. Date of Earliest Transaction (Month/Day/Year) 03/11/2013 (Street) WASHINGTON, DC 20005 4. If Amendment, Date Original Filed(Month/Day/Year) 6. Individual or Joint/Group Filing(Check Applicable Line) _X_ Form filed by One Reporting Person ___ Form filed by More than One Reporting Person (City) (State) (Zip) Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned

1.Title of Security (Instr. 3) 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock, par value $0.01 per share 03/11/2013   A   6,000 A $ 0 50,184 (1) D   Common Stock, par value $0.01 per share 03/12/2013   M   2,300 A $ 42.29 52,484 (2) D   Common Stock, par value $0.01 per share 03/12/2013   M   3,900 A $ 57.16 56,384 (3) D   Common Stock, par value $0.01 per share 03/12/2013   M   196 A $ 39.53 56,580 (4) D   Common Stock, par value $0.01 per share 03/12/2013   M   3,866 A $ 58.95 60,446 (5) D   Common Stock, par value $0.01 per share 03/12/2013   S   14,599 D $ 101.1 (6) 45,847 (7) D   Common Stock, par value $0.01 per share 03/12/2013   F   486 D $ 101.06 45,361 (8) D

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Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)

1. Title of Derivative Security (Instr. 3)

2. Conversion or Exercise Price of Derivative Security

3. Transaction Date (Month/Day/Year)

3A. Deemed Execution Date, if any (Month/Day/Year)

4. Transaction Code (Instr. 8)

5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5)

6. Date Exercisable and Expiration Date (Month/Day/Year)

7. Title and Amount of Underlying Securities (Instr. 3 and 4)

8. Price of Derivative Security (Instr. 5)

9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)

10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4)

11. Nature of Indirect Beneficial Ownership (Instr. 4)

Code

V

(A)

(D)

Date Exercisable

Expiration Date

Title

Amount or Number of Shares

Option to Acquire Common Stock

$ 102.16

03/11/2013

A

10,100

(9)

03/10/2023

Common Stock

10,100

$ 0

10,100

D

Option to Acquire Common Stock

$ 42.29

03/12/2013

M

2,300

(10)

03/11/2020

Common Stock

2,300

$ 0

0

D

Option to Acquire Common Stock

$ 57.16

03/12/2013

M

3,900

(11)

03/03/2021

Common Stock

3,900

$ 0

3,900

D

Option to Acquire Common Stock

$ 39.53

03/12/2013

M

196

(12)

05/04/2014

Common Stock

196

$ 0

0

D

Option to Acquire Common Stock

$ 58.95

03/12/2013

M

3,866

(13)

02/20/2022

Common Stock

3,866

$ 0

7,734

D

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
Stanfill John L C/O COSTAR GROUP, INC. 1331 L STREET, NW WASHINGTON, DC 20005			Sr. VP-Sales, Customer Service

Signatures

/s/ Jonathan Coleman, Attorney-in-Fact		03/13/2013
**Signature of Reporting Person		Date

Explanation of Responses:

*	If the form is filed by more than one reporting person, see Instruction 4(b)(v).
**	Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1)	Represents all shares of Common Stock owned, consisting of 5,449 shares of Common Stock and 44,735 shares of Common Stock subject to unvested Restricted Stock Grants.
(2)	Represents all shares of Common Stock owned, consisting of 7,749 shares of Common Stock and 44,735 shares of Common Stock subject to unvested Restricted Stock Grants.
(3)	Represents all shares of Common Stock owned, consisting of 11,649 shares of Common Stock and 44,735 shares of Common Stock subject to unvested Restricted Stock Grants.
(4)	Represents all shares of Common Stock owned, consisting of 11,845 shares of Common Stock and 44,735 shares of Common Stock subject to unvested Restricted Stock Grants.
(5)	Represents all shares of Common Stock owned, consisting of 15,711 shares of Common Stock and 44,735 shares of Common Stock subject to unvested Restricted Stock Grants.
(6)	Average based on sales prices ranging from $100.75 to $102.35.
(7)	Represents all shares of Common Stock owned, consisting of 1,112 shares of Common Stock and 44,735 shares of Common Stock subject to unvested Restricted Stock Grants.
(8)	Represents all shares of Common Stock owned, consisting of 1,660 shares of Common Stock and 43,701 shares of Common Stock subject to unvested Restricted Stock Grants.
(9)	The option vests in three equal installments on March 11, 2014, March 11, 2015, and March 11, 2016.
(10)	The option vested in three equal installments on March 12, 2011, March 12, 2012, and March 12, 2013.
(11)	The option vests in three equal installments on March 4, 2012, March 4, 2013, and March 4, 2014.
(12)	The option vested in three equal installments on May 5, 2005, May 5, 2006, May 5, 2007, and May 5, 2008.
(13)	The option vests in three equal installments on February 21, 2013, February 21, 2014, and February 21, 2015.

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