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 August, 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 2,700,000 ordinary shares at  an average price of 62.72 pence per
share. The highest price and the lowest price paid for these shares were 63.0
pence  and 62.0 pence respectively.  Spirent Communications plc intends to hold
the purchased shares in treasury.

Following the purchase of these shares, Spirent Communications plc holds
79,445,000 of its ordinary shares  in treasury and has 884,586,304 ordinary
shares in issue (excluding treasury shares). This figure (884,586,304)
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 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: 29 August 2007                     By   ____/s/ Michael Anscombe____

                                                    (Signature)*