<PAGE>   1

               UNITED STATES SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON DC  20549

                                 FORM 10-Q/A

                               AMENDMENT NO. 1

                                      TO
(Mark One)

/X/      QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF
         THE SECURITIES EXCHANGE ACT OF 1934
         For the quarterly period ended September 30, 1994

                                      OR

/ /      TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
         EXCHANGE ACT OF 1934 FOR THE TRANSITION PERIOD FROM
                              TO 
         --------------------    --------------------
        
Commission file number 0-9992


                         KLA INSTRUMENTS CORPORATION
            (Exact name of registrant as specified in its charter)

             Delaware                                 04-2564110
    (State of incorporation)             (IRS Employer Identification Number)

                                160 Rio Robles
                          San Jose, California 95134
                                (408) 434-4200

        (Address and telephone number of principal executive offices)

                               ---------------

         Indicate by check mark whether registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to such
filing requirements for the past 90 days.


                                 YES  /X/     NO
                                     ----        ----

         SECURITIES REGISTERED PURSUANT TO SECTION 12(G) OF THE ACT:
                        Common Stock, $0.001 par value
                         Common Stock Purchase Rights

         Common shares outstanding at September 30, 1994:   23,054,000

         THIS AMENDMENT 10-Q/A CONTAINS 4 PAGES.

<PAGE>   2

                             FORM 10-Q/A AMENDMENT


         The registrant hereby amends Part II Item 6 EXHIBITS AND REPORTS ON
FORM 8-K, by filing herewith Exhibit 27, 1995 First Quarter Financial Data
Schedule.





                                       2

<PAGE>   3

                                  SIGNATURES


         Pursuant to the requirements of the Securities Exchange Act of 1934,
the registrant has duly caused this amendment to be signed on its behalf by the
undersigned, thereunto duly authorized.

                                        KLA INSTRUMENTS CORPORATION



                                        By:  /s/ William Turner
                                             -----------------------------------
                                             William Turner
                                             Vice President/Corporate Controller
Date:  December 15, 1994





                                       3






<TABLE> <S> <C>

<ARTICLE> 5
<LEGEND>
THIS SCHEDULE CONTAINS SUMMARY FINANCIAL INFORMATION EXTRACTED FROM THE
CONSOLIDATED STATEMENT OF OPERATIONS, THE CONSOLIDATED BALANCE SHEET AND THE
ACCOMPANYING NOTES TO THE CONSOLIDATED FINANCIAL STATEMENTS, AND IS QUALIFIED IN
ITS ENTIRETY BY REFERENCE TO SUCH FINANCIAL STATEMENTS.
</LEGEND>

       
<S>                               <C>                     <C>
<MULTIPLIER>                               1000
<PERIOD-TYPE>                             QTR-1                   3-MOS
<FISCAL-YEAR-END>                   JUN-30-1995             JUN-30-1995
<PERIOD-START>                      JUL-01-1994             JUL-01-1994
<PERIOD-END>                        SEP-30-1994             SEP-30-1994
<CASH>                                  144,073                 144,073
<SECURITIES>                                  0                       0
<RECEIVABLES>                            85,793                  85,793
<ALLOWANCES>                              1,787                   1,787
<INVENTORY>                              59,955                  59,955
<CURRENT-ASSETS>                        300,252                 300,252
<PP&E>                                   76,792                  76,792
<DEPRECIATION>                           38,526                  38,526
<TOTAL-ASSETS>                          344,619                 344,619
<CURRENT-LIABILITIES>                    94,054                  94,054
<BONDS>                                       0                       0
<COMMON>                                     23                      23
<PREFERRED-MANDATORY>                         0                       0
<PREFERRED>                                   0                       0
<OTHER-SE>                              241,936                 241,936
<TOTAL-LIABILITY-AND-EQUITY>            344,619                 344,619
<SALES>                                  83,179                  83,179
<TOTAL-REVENUES>                         83,179                  83,179
<CGS>                                    40,602                  40,602
<TOTAL-COSTS>                            65,234                  65,234
<OTHER-EXPENSES>                              0                       0
<LOSS-PROVISION>                              0                       0
<INTEREST-EXPENSE>                          474                     474
<INCOME-PRETAX>                          18,968                  18,968
<INCOME-TAX>                              6,070                   6,070
<INCOME-CONTINUING>                      12,898                  12,898
<DISCONTINUED>                                0                       0
<EXTRAORDINARY>                               0                       0
<CHANGES>                                     0                       0
<NET-INCOME>                             12,898                  12,898
<EPS-PRIMARY>                               .54                     .54
<EPS-DILUTED>                               .54                     .54
                               
                               

</TABLE>