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 September, 2006

                                 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
Citigroup Global Markets U.K. Equity Limited 1,000,000 ordinary shares at an
average price of  50.4723 pence per share. The highest price and the lowest
price paid for these shares were  51.0 pence and  50.0 pence respectively.

Following the above purchase, Spirent Communications plc holds 68,390,000
Spirent Communications plc ordinary shares in treasury, and has 904,014,372
Spirent Communications plc ordinary shares in issue (excluding treasury shares).

                                   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: 27 September, 2006                    By   ____/s/ Michael Anscombe____

                                                    (Signature)*