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                                                           Exhibit 31(a)
                              CERTIFICATION
                              _____________

I, Dennis J. Kuester, Chairman of the Board and Chief Executive Officer
of Marshall & Ilsley Corporation, certify that:

     1.   I have reviewed this quarterly report on Form 10-Q of
          Marshall & Ilsley 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)) and internal control over financial
          reporting (as defined in Exchange Act Rules 13a-15(f) and
          15d-15(f)) 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;

          (b)  Designed such internal control over financial
               reporting, or caused such internal control over
               financial reporting to be designed under our
               supervision, to provide reasonable assurance regarding
               the reliability of financial reporting and the
               preparation of financial statements for external
               purposes in accordance with generally accepted
               accounting principles;

          (c)  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

          (d)  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

     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

          (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:  May 10, 2006

/s/ Dennis J. Kuester
_________________________________________________

Dennis J. Kuester
Chairman of the Board and Chief Executive Officer


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