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 March, 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 on 27 March 2007 it purchased through
ABN AMRO Bank NV 280,000  ordinary shares at an average price of 61.69 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
70,605,000  of its  ordinary  shares in treasury  and has  886,254,230  ordinary
shares  in  issue  (excluding  treasury  shares).   This  figure   (886,254,230)
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: 28 March, 2007                      By   ____/s/ Michael Anscombe____

                                                    (Signature)*