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|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 Annual Report of Castelle (the Company) on Form 10-K for the period
ended December 31, 2006, 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. § 1350, as adopted pursuant to § 906 of the Sarbanes-Oxley Act of
2002, that to my knowledge:
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| | (1) | The Report fully complies with the requirements of section 13(a) or 15(d) of the Securities Exchange
  Act of 1934; and                               
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| | (2) | The information contained in the Report fairly presents, in all material respects, the financial condition
  and results of operations of the Company.
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| | /s/ Scott C.
McDonaldName:     Scott C. McDonald  
| | --------------------------------------------------
| | Title:       President and Chief Executive Officer
| |                                                   
| | Date:       March 30, 2007                        

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|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.

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