UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                    FORM 6-K

                        REPORT OF FOREIGN PRIVATE ISSUER
                        PURSUANT TO RULE 13a-16 OR 15d-16
                    UNDER THE SECURITIES EXCHANGE ACT OF 1934




                          For the month of April, 2007

                           SPIRENT COMMUNICATIONS plc
      _____________________________________________________________________
                 (Translation of registrant's name into English)

   Spirent House, Crawley Business Quarter, Fleming Way, Crawley, West Sussex
                                  RH10 9QL, UK.
      _____________________________________________________________________
                    (Address of principal executive offices)


Indicate by check mark whether the registrant files or will file annual reports
under cover Form 20-F or Form 40-F.

                         Form 20-F    X         Form 40-F.....

Indicate by check mark whether the registrant by furnishing the information
contained in this Form is also thereby furnishing the information to the
Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934.

                               Yes .....        No     X

If "Yes" is marked, indicate below the file number assigned to the registrant
in connection with Rule 12g3-2(b): 82- ________



Spirent Communications plc: Transaction in own shares

Spirent Communications plc announces that it has today purchased through ABN
AMRO Bank NV 500,000 ordinary shares at an  average price of 61.9 pence per
share. The highest price and the lowest price paid for these shares were 62
pence and  61.5 pence respectively.

Spirent Communications plc intends to hold the purchased shares in treasury.

Following the purchase of these shares, Spirent Communications plc holds
74,760,000 of its ordinary shares in treasury and has 882,154,169 ordinary
shares in issue (excluding treasury shares). This figure (882,154,169)
represents the total voting rights in the Company and may be used by
shareholders as the denominator for the calculations by which they can determine
if they are required to notify their interest in, or a change to their interest
in, the Company under the Financial Services Authority's Disclosure and
Transparency Rules.



                                   Signatures

Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned, thereunto duly authorized.

                                               ______MICHAEL ANSCOMBE______

                                                     (Registrant)

Date: April 12, 2007                      By   ____/s/ Michael Anscombe____

                                                    (Signature)*