EXHIBIT 31.1

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

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

2.       
          Based on my knowledge, this 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
          report;

3.       
          Based on my knowledge, the financial statements, and other financial information
          included in this report, fairly present in all material respects the
          financial condition, results of operations and cash flows of the registrant as
          of, and for, the periods presented in this 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-15(e) and 15d-15(e)) 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 report is being prepared;
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| | b) | Evaluated the effectiveness of the registrants disclosure controls and procedures and presented in this report our conclusions about
the effectiveness of the disclosure controls and procedures, as of the end of the period covered by this report based on such
evaluation; and
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| | c) | Disclosed in this 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; and
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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 the 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
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| | 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.
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| |                           
| | --------------------------
| | Date: October 31, 2003    
| |                           
| | /s/ JOSEPH D. KEEGAN, PH.D
| |                           
| | Joseph D. Keegan, Ph.D.   
| | Chief Executive Officer