Exhibit 31(a)Certification Pursuant to Rule 13a-14(a)/15d-14(a)I, Joseph P. Keithley, certify that: 1. I have reviewed this quarterly report on Form 10-Q of Keithley Instruments, Inc.;  2. Based on my knowledge, this quarterly report does not contain any untrue statement of amaterial fact or omit to state a material fact necessary in order to make the statementsmade, in light of the circumstances under which such statements were made, not misleadingwith respect to the period covered by this quarterly report;  3. Based on my knowledge, the financial statements, and other financial informationincluded in this quarterly report, fairly present in all material respects the financialcondition, results of operations and cash flows of the registrant as of, and for, theperiods presented in this quarterly report;  4. The registrant’s other certifying officers and I are responsible for establishing andmaintaining 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 ActRules 13a-15(f) and 15d-15(f)) for the registrant and have: a) designed such disclosure controls and procedures, or caused suchdisclosure controls and procedures to be designed under our supervision, to ensurethat material information relating to the registrant, including its consolidatedsubsidiaries, is made known to us by others within those entities, particularlyduring the period in which this quarterly report is being prepared;  b) designed such internal control over financial reporting, or caused suchinternal control over financial reporting to be designed under our supervision, toprovide reasonable assurance regarding the reliability of financial reporting andthe preparation of financial statements for external purposes in accordance withgenerally accepted accounting principles;  c) evaluated the effectiveness of the registrant’s disclosure controls andprocedures and presented in this quarterly report our conclusions about theeffectiveness of the disclosure controls and procedures, as of the end of theperiod covered by this quarterly report based on such evaluation; and  d) disclosed in this quarterly report any change in the registrant’sinternal control over financial reporting that occurred during the registrant’smost recent fiscal quarter (the registrant’s fourth fiscal quarter in the case ofan annual report) that has materially affected, or is reasonably likely tomaterially affect, the registrant’s internal control over financial reporting; and 5. The registrant’s other certifying officers and I have disclosed, based on our mostrecent evaluation of internal control over financial reporting, to the registrant’sauditors and the audit committee of the registrant’s board of directors (or personsperforming the equivalent functions): a) all significant deficiencies and material weaknesses in the design oroperation of internal control over financial reporting which are reasonably likelyto adversely affect the registrant’s ability to record, process, summarize, andreport financial information; and  b) any fraud, whether or not material, that involves management or otheremployees who have a significant role in the registrant’s internal control overfinancial reporting.        Date: May 10, 2006 /s/ Joseph P. Keithley   Chairman, President and Chief Executive Officer