Exhibit 32.1

                Certification Pursuant to 18 U.S.C. Section 1350,
                             As Adopted Pursuant To
                  Section 906 of the Sarbanes-Oxley Act of 2002



         In connection with the Quarterly Report of Castelle (the "Company") on
Form 10-Q for the period ended March 31, 2004, as filed with the Securities and
Exchange Commission on the date hereof (the "Report"), I, Scott C. McDonald,
Chief Executive Officer of the Company, certify, pursuant to 18 U.S.C. ss. 1350,
as adopted pursuant to ss. 906 of the Sarbanes-Oxley Act of 2002, that to my
knowledge:

     (1)  The Report fully  complies with the  requirements  of Section 13(a) of
          the Securities Exchange Act of 1934; and

     (2)  The  information  contained  in the  Report  fairly  presents,  in all
          material respects,  the financial  condition and results of operations
          of the Company.



                  /s/ Scott C. McDonald
         Name:    Scott C. McDonald
         Title:   Chief Executive Officer

         Date:    May 11, 2004

This certification accompanies this report pursuant to Section 906 of the
Sarbanes-Oxley Act of 2002 and shall not be deemed to be filed by Castelle for
purposes of Section 18 of the Securities Exchange Act of 1934. A signed original
of this written statement required by Section 906 has been provided to Castelle
and will be retained by Castelle and furnished to the Securities and Exchange
Commission or its staff upon request.

                                      E-3