UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                  SCHEDULE 13G

                    Under the Securities Exchange Act of 1934

                                (Amendment No. )*

                                    RLI Corp.
--------------------------------------------------------------------------------
                                (Name of Issuer)

                                  Common Stock
--------------------------------------------------------------------------------
                         (Title of Class of Securities)

                                    749607107
                  ---------------------------------------------
                                 (CUSIP Number)

                                December 31, 2002
         --------------------------------------------------------------
             (Date of Event Which Requires Filing of this Statement)

     Check the appropriate box to designate the rule pursuant to which this
                               Schedule is filed:

                                [x] Rule 13d-1(b)
                                [ ] Rule 13d-1(c)
                                [ ] Rule 13d-1(d)

*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter the
disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).


                               Page 1 of 10 pages



---------------------------                                   ------------------
CUSIP No. 749607107                   13G                     Page 2 of 10 Pages
---------------------------                                   ------------------

--------------------------------------------------------------------------------
 1     NAME OF REPORTING PERSON
       S.S. or I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

           Liberty Wanger Asset Management, L.P.  36-3820584
--------------------------------------------------------------------------------
 2     CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

           Not Applicable                                               (a) [ ]

                                                                        (b) [ ]
--------------------------------------------------------------------------------
 3     SEC USE ONLY

--------------------------------------------------------------------------------
 4     CITIZENSHIP OR PLACE OF ORGANIZATION

           Delaware
--------------------------------------------------------------------------------
                       5    SOLE VOTING POWER
    NUMBER OF
                                None
      SHARES           ---------------------------------------------------------
                            SHARED VOTING POWER
   BENEFICIALLY
                       6        1,030,000
     OWNED BY          ---------------------------------------------------------
                            SOLE DISPOSITIVE POWER
       EACH
                       7        None
    REPORTING          ---------------------------------------------------------

   PERSON WITH         8    SHARED DISPOSITIVE POWER

                                1,030,000
--------------------------------------------------------------------------------
9     AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

                       1,030,000
--------------------------------------------------------------------------------
10   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*

                       Not Applicable                                       [ ]
--------------------------------------------------------------------------------
11   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

                       5.2%
--------------------------------------------------------------------------------
12   TYPE OF REPORTING PERSON*

                       IA
--------------------------------------------------------------------------------

                      *SEE INSTRUCTION BEFORE FILLING OUT!



---------------------------                                   ------------------
CUSIP No. 749607107                   13G                     Page 3 of 10 Pages
---------------------------                                   ------------------

--------------------------------------------------------------------------------
 1     NAME OF REPORTING PERSON
       S.S. or I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

           WAM Acquisition GP, Inc.
--------------------------------------------------------------------------------
 2     CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

           Not Applicable                                               (a) [ ]

                                                                        (b) [ ]
--------------------------------------------------------------------------------
 3     SEC USE ONLY

--------------------------------------------------------------------------------
 4     CITIZENSHIP OR PLACE OF ORGANIZATION

           Delaware
--------------------------------------------------------------------------------
                       5    SOLE VOTING POWER
    NUMBER OF
                                None
      SHARES           ---------------------------------------------------------
                            SHARED VOTING POWER
   BENEFICIALLY
                       6        1,030,000
     OWNED BY          ---------------------------------------------------------
                            SOLE DISPOSITIVE POWER
       EACH
                       7        None
    REPORTING          ---------------------------------------------------------

   PERSON WITH         8    SHARED DISPOSITIVE POWER

                                1,030,000
--------------------------------------------------------------------------------
9     AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

                       1,030,000
--------------------------------------------------------------------------------
10   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*

                       Not Applicable                                       [ ]
--------------------------------------------------------------------------------
11   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

                       5.2%
--------------------------------------------------------------------------------
12   TYPE OF REPORTING PERSON*

                       CO
--------------------------------------------------------------------------------

                      *SEE INSTRUCTION BEFORE FILLING OUT!



---------------------------                                   ------------------
CUSIP No. 749607107                   13G                     Page 4 of 10 Pages
---------------------------                                   ------------------

--------------------------------------------------------------------------------
 1     NAME OF REPORTING PERSON
       S.S. or I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

           Liberty Acorn Trust
--------------------------------------------------------------------------------
 2     CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

           Not Applicable                                               (a) [ ]

                                                                        (b) [ ]
--------------------------------------------------------------------------------
 3     SEC USE ONLY

--------------------------------------------------------------------------------
 4     CITIZENSHIP OR PLACE OF ORGANIZATION

            Massachusetts
--------------------------------------------------------------------------------
                       5    SOLE VOTING POWER
    NUMBER OF
                                None
      SHARES           ---------------------------------------------------------
                            SHARED VOTING POWER
   BENEFICIALLY
                       6        1,000,000
     OWNED BY          ---------------------------------------------------------
                            SOLE DISPOSITIVE POWER
       EACH
                       7        None
    REPORTING          ---------------------------------------------------------

   PERSON WITH         8    SHARED DISPOSITIVE POWER

                                1,000,000
--------------------------------------------------------------------------------
9     AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

                       1,000,000
--------------------------------------------------------------------------------
10   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*

                       Not Applicable                                       [ ]
--------------------------------------------------------------------------------
11   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

                       5.0%
--------------------------------------------------------------------------------
12   TYPE OF REPORTING PERSON*

                       IV
--------------------------------------------------------------------------------

                      *SEE INSTRUCTION BEFORE FILLING OUT!


Item 1(a)                  Name of Issuer:

                                   RLI Corp.

Item 1(b)                  Address of Issuer's Principal Executive Offices:

                                   9025 North Lindbergh Drive
                                   Peoria, IL 61615

Item 2(a)                  Name of Person Filing:

                                   Liberty Wanger Asset Management, L.P. ("WAM")
                                   WAM Acquisition GP, Inc., the general
                                   partner of WAM ("WAM GP")
                                   Liberty Acorn Trust ("Acorn")

Item 2(b)                  Address of Principal Business Office:

                                   WAM, WAM GP and Acorn are all located at:

                                   227 West Monroe Street, Suite 3000
                                   Chicago, Illinois  60606

Item 2(c)                  Citizenship:

                                   WAM is a Delaware limited partnership; WAM
                                   GP is a Delaware corporation; and Acorn is a
                                   Massachusetts business trust.

Item 2(d)                  Title of Class of Securities:

                                   Common Stock

Item 2(e)                  CUSIP Number:

                                   749607107

Item 3                     Type of Person:

                                   (d)     Acorn is an Investment Company under
                                           section 8 of the Investment Company
                                           Act.

                                   (e)     WAM is an Investment Adviser
                                           registered under section 203 of the
                                           Investment Advisers Act of 1940; WAM
                                           GP is the General Partner of the
                                           Investment Adviser.


                               Page 5 of 10 pages


Item 4                     Ownership (at December 31, 2002):

                                    (a)     Amount owned "beneficially" within
                                            the meaning of rule 13d-3:

                                            1,030,000

                                    (b)     Percent of class:

                                            5.2% (based on 19,875,631 shares
                                            outstanding as of October 25, 2002,
                                            based on Form 10-K filed on December
                                            20, 2002).

                                    (c)     Number of shares as to which such
                                            person has:

                                                  (i)     sole power to vote or
                                                          to direct the vote:
                                                          none

                                                 (ii)     shared power to vote
                                                          or to direct the vote:
                                                          1,030,000

                                                (iii)     sole power to dispose
                                                          or to direct the
                                                          disposition of: none

                                                 (iv)     shared power to
                                                          dispose or to direct
                                                          disposition of:
                                                          1,030,000

Item 5                     Ownership of Five Percent or Less of a Class:

                                    Not Applicable

Item 6                     Ownership of More than Five Percent on Behalf of
                           Another Person:

                                    The shares reported herein have been
                                    acquired on behalf of discretionary clients
                                    of WAM, including Acorn. Persons other than
                                    WAM and WAM GP are entitled to receive all
                                    dividends from, and proceeds from the sale
                                    of, those shares. Acorn is the only such
                                    person known to be entitled to all dividends
                                    from, and all proceeds from the sale of,
                                    shares reported herein to the extent of more
                                    than 5% of the class.

Item 7                     Identification and Classification of the Subsidiary
                           Which Acquired the Security Being Reported on by the
                           Parent Holding Company:

                                    Not Applicable

Item 8                     Identification and Classification of Members of the
                           Group:

                                    Not Applicable

Item 9                     Notice of Dissolution of Group:

                                    Not Applicable


                               Page 6 of 10 Pages


Item 10                    Certification:

                                    By signing below I certify that, to the
                           best of my knowledge and belief, the securities
                           referred to above were acquired and are held in the
                           ordinary course of business and were not acquired and
                           are not held for the purpose of or with the effect of
                           changing or influencing the control of the issuer of
                           the securities and were not acquired and are not held
                           in connection with or as a participant in any
                           transaction having that purpose or effect.


                               Page 7 of 10 Pages


                                    Signature

            After reasonable inquiry and to the best of my knowledge and belief,
I certify that the information set forth in this statement is true, complete and
correct.

Date: February 14, 2003

                           The undersigned corporation, on the date above
                           written, agrees and consents to the joint filing on
                           its behalf of this Schedule 13G in connection with
                           its beneficial ownership of the security reported
                           herein.

                                  WAM Acquisition GP, Inc.
                                      for itself and as general partner of
                                      LIBERTY WANGER ASSET MANAGEMENT, L.P.

                                  By:  /s/ Bruce H. Lauer
                                       -----------------------------------------
                                           Bruce H. Lauer
                                           Senior Vice President and Secretary

                           The undersigned corporation, on the date above
                           written, agrees and consents to the joint filing on
                           its behalf of this Schedule 13G in connection with
                           its beneficial ownership of the security reported
                           herein.

                                  LIBERTY ACORN TRUST

                                  By:  /s/ Bruce H. Lauer
                                       -----------------------------------------
                                           Bruce H. Lauer
                                           Vice President, Treasurer and
                                           Secretary

                               Page 8 of 10 Pages



                                  Exhibit Index

Exhibit 1                  Joint Filing Agreement dated as of February 14, 2003
                           by and among Liberty Wanger Asset Management, L.P.,
                           WAM Acquisition GP, Inc. and Liberty Acorn Trust.


                               Page 9 of 10 Pages