Castelle 10-Q 2006 Q1 Quarterly report

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, 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 the best of 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 and President

Date: May 12, 2006

This certification accompanies the Report pursuant to Section 906 of the Sarbanes-Oxley Act of 2002 and shall not be deemed to be filed by the Company 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 the Company and will be retained by the Company and furnished to the Securities and Exchange Commission or its staff upon request.

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