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                                                                   Exhibit 31.1

I, Joseph D. Keegan, Ph.D., certify that:

1.      I have reviewed this annual report on Form 10-K of Molecular Devices
Corporation;

2.      Based on my knowledge, this annual report does not contain any untrue
statement of a material fact or omit to state a material fact necessary to make
the statements made, in light of the circumstances under which such statements
were made, not misleading with respect to the period covered by this annual
report;

3.      Based on my knowledge, the consolidated financial statements, and other
financial information included in this annual report, fairly present in all
material respects the consolidated financial condition, results of operations
and cash flows of the registrant as of, and for, the periods presented in this
annual report;


4.      The registrant's other certifying officer(s) and I are responsible for
establishing and maintaining disclosure controls and procedures (as defined in
Exchange Act Rules 13a-14(a) and 15d-14(a)) for the registrant and have:

         a) designed such disclosure controls and procedures, or caused such
         disclosure controls and procedures to be designed under our
         supervision, to ensure that material information relating to the
         registrant, including its consolidated subsidiaries, is made known to
         us by others within those entities, particularly during the period in
         which this annual report is being prepared;

         b) evaluated the effectiveness of the registrant's disclosure controls
         and procedures and presented in this annual report our conclusions
         about the effectiveness of the disclosure controls and procedures, as
         of the end of the period covered by this annual report based on such
         evaluation; and

         c)disclosed in this annual report any change in the registrant's
         internal control over financial reporting that occurred during the
         registrant's most recent fiscal quarter (the registrant's fourth
         fiscal quarter in the case of an annual report) that has materially
         affected, or is reasonably likely to materially affect, the
         registrant's internal control over financial reporting.

5.      The registrant's other certifying officer(s) and I have disclosed, based
on our most recent evaluation of internal control over financial reporting, to
the registrant's auditors and the audit committee of registrant's board of
directors (or persons performing the equivalent functions):

         a) all significant deficiencies and material weaknesses in the design
         or operation of internal control over financial reporting which are
         reasonably likely to adversely affect the registrant's ability to
         record, process, summarize and report financial information; and

         b) any fraud, whether or not material, that involves management or
         other employees who have a significant role in the registrant's
         internal control over financial reporting.


Date:  March 9, 2004

/s/ Joseph D. Keegan
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Joseph D. Keegan, Ph.D.
Chief Executive Officer