<PAGE>

                      SECURITIES AND EXCHANGE COMMISSION
                             Washington, DC  20549
                                       
                                       
                                 SCHEDULE 13G
                                (Rule 13d-102)
                                       
                                       
            INFORMATION TO BE INCLUDED IN STATEMENTS FILED PURSUANT
          TO RULES 13d-1(b)(c), AND (d) AND AMENDMENTS THERETO FILED
                           PURSUANT TO RULE 13d-2(b)
                            (Amendment No.    )(1)
                                          ----
                                       
                                       
                                  Itron, Inc.
 -----------------------------------------------------------------------------
                               (Name of Issuer)
                                       
                                 Common Stock
------------------------------------------------------------------------------
                        (Title of Class of Securities)
                                       
                                   465741106
 -----------------------------------------------------------------------------
                                 (CUSIP Number)
                                       
                                  12/31/1999
 -----------------------------------------------------------------------------
            (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)


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

(1)  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 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>
---------------------                                    ---------------------
CUSIP No. 465741106                  13G                 Page 2 of 8 Pages
---------------------                                    ---------------------

------------------------------------------------------------------------------
1. NAMES OF REPORTING PERSONS
   I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS (ENTITIES ONLY)

          The TCW Group, Inc.
------------------------------------------------------------------------------
2. CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                (a)   / /
                                                                (b)   /X/
------------------------------------------------------------------------------
3. SEC USE ONLY


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

          Nevada corporation
------------------------------------------------------------------------------
           NUMBER OF      5. SOLE VOTING POWER
            SHARES                                  -0-
         BENEFICIALLY   ------------------------------------------------------
           OWNED BY       6. SHARED VOTING POWER
             EACH                                   966,300
           REPORTING    ------------------------------------------------------
            PERSON        7. SOLE DISPOSITIVE POWER
             WITH                                   -0-
                        ------------------------------------------------------
                          8. SHARED DISPOSITIVE POWER
                                                    966,300
------------------------------------------------------------------------------
9. AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
          966,300
------------------------------------------------------------------------------
10. CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*
                                                                  / /
------------------------------------------------------------------------------
11. PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)
           6.5% (see response to Item 4)
------------------------------------------------------------------------------
12. TYPE OF REPORTING PERSON*
          HC/CO
------------------------------------------------------------------------------




                     *SEE INSTRUCTIONS BEFORE FILLING OUT


<PAGE>
---------------------                                    ---------------------
CUSIP No. 465741106                  13G                 Page 3 of 8 Pages
---------------------                                    ---------------------

------------------------------------------------------------------------------
1. NAMES OF REPORTING PERSONS
   I.R.S. IDENTIFICATION NO. OF ABOVE PERSONS (ENTITIES ONLY)

          Robert Day
------------------------------------------------------------------------------
2. CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                             (a)   / /
                                                             (b)   /X/
------------------------------------------------------------------------------
3. SEC USE ONLY


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

          United States Citizen
------------------------------------------------------------------------------
           NUMBER OF      5. SOLE VOTING POWER
            SHARES                                    -0-
         BENEFICIALLY   ------------------------------------------------------
           OWNED BY       6. SHARED VOTING POWER
             EACH                                     966,300
           REPORTING    ------------------------------------------------------
            PERSON        7. SOLE DISPOSITIVE POWER
             WITH                                     -0-
                        ------------------------------------------------------
                          8. SHARED DISPOSITIVE POWER
                                                      966,300
------------------------------------------------------------------------------
9. AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

          966,300
------------------------------------------------------------------------------
10. CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*
                                                                   / /
------------------------------------------------------------------------------
11. PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)

           6.5% (see response to Item 4)
------------------------------------------------------------------------------
12. TYPE OF REPORTING PERSON*

          HC/IN
------------------------------------------------------------------------------




                     *SEE INSTRUCTIONS BEFORE FILLING OUT



<PAGE>

                                                            Page 4 of 8 Pages

Item 1(a).     Name of Issuer:

               Itron, Inc.

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

               2818 N. Sullivan Rd.
               Spokane, WA 99216

Item 2(a).     Name of Persons Filing:
Item 2(b).     Address of Principal Business Office, or if None, Residence:
Item 2(c).     Citizenship:

               The TCW Group, Inc.
               865 South Figueroa Street
               Los Angeles, CA 90017
               (Nevada Corporation)

               Robert Day
               865 South Figueroa Street
               Los Angeles, CA 90017
               (United States Citizen)

Item 2(d).     Title of Class of Securities:

               Common Stock

Item 2(e).     CUSIP Number:

               465741106


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                                                             Page 5 of 8 Pages


Item 3.   If This Statement Is Filed Pursuant to Rule 13d-1(b), or 13d-2(b) or
          (c), Check Whether the Person Filing is a:

          (a)  / /  Broker or dealer registered under Section 15 of the
                    Exchange Act.
          (b)  / /  Bank as defined in Section 3(a)(6) of the Exchange Act.
          (c)  / /  Insurance company as defined in Section 3(a)(19) of the
                    Exchange Act.
          (d)  / /  Investment company registered under Section 8 of the
                    Investment Company Act.
          (e)  / /  An investment adviser in accordance with Rule
                    13d-1(b)(1)(ii)(E).
          (f)  / /  An employee benefit plan or endowment fund in
                    accordance with 13d-1(b)(1)(ii)(F).
          (g)  /X/  A parent holding company or control person in
                    accordance with Rule 13d-1(b)(1)(ii)(G).

                    (SEE Item 7)
                    The TCW Group, Inc.
                    Robert Day (individual who may be deemed to control The
                         TCW Group, Inc. and other entities which hold the
                         Common Stock of the issuer)

          (h)  / /  A savings association as defined in Section 3(b) of the
                    Federal Deposit Insurance Act.
          (i)  / /  A church plan that is excluded from the definition of
                    an investment company under Section 3(c)(14) of the
                    Investment Company Act.
          (j)  / /  Group, in accordance with Rule 13d-1(b)(1)(ii)(J).
          If this statement is filed pursuant to Rule 13d-1(c), check this
          box. / /


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                                                             Page 6 of 8 Pages

Item 4.   Ownership **

          THE TCW GROUP, INC.
          -------------------
          (a)  Amount beneficially owned: 966,300
          (b)  Percent of class:  6.5%
          (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:
                       966,300
                (iii)  Sole power to dispose or to direct the disposition
                       of: none.
                (iv)   Shared power to dispose or to direct the disposition
                       of: 966,300

          ROBERT DAY ***
          ----------
          (a)  Amount beneficially owned: 966,300
          (b)  Percent of class:  6.5%
          (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:
                     966,300
              (iii)  Sole power to dispose or direct the disposition of:
                     none.
              (iv)   Shared power to dispose or to direct the disposition of:
                     966,300











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

**   The filing of this Schedule 13G shall not be construed as an admission
     that the reporting person or any of its affiliates is, for the purposes
     of Section 13(d) or 13(g) of the Securities Exchange Act of 1934, the
     beneficial owner of any securities covered by this Schedule 13G.  In
     addition, the filing of this Schedule 13G shall not be construed as an
     admission that the reporting person or any of its affiliates is the
     beneficial owner of any securities covered by this Schedule 13G for any
     other purposes than Section 13(d) of the Securities Exchange Act of 1934.

***  Shares reported for Robert Day include shares reported for The TCW Group,
     Inc.


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                                                             Page 7 of 8 Pages


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

          If this statement is being filed to report the fact that as of the
          date hereof the reporting person has ceased to be the beneficial
          owner of more than five percent of the class of securities, check
          the following / /.

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

          Various persons other than as described in Item 4 have the right to
     receive or the power to direct the receipt of dividends from, or the
     proceeds from the sale of, the Common Stock of Itron, Inc.

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

          SEE Exhibit A.

Item 8.   Identification and Classification of Members of the Group.

          Not applicable.  SEE Exhibits A and B.

Item 9.   Notice of Dissolution of Group.

          Not applicable.

Item 10.  Certification.

          Because this statement is filed pursuant to Rule 13d-1(b), the
          following certification is included:

          By signing below I certify that, to the best of my knowledge and
          belief, the securities referred to above were acquired 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.


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                                                             Page 8 of 8 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.


Dated this 11th day of February, 2000.


                                        The TCW Group, Inc.

                                        By: /s/ Philip K. Holl
                                            --------------------
                                            Philip K. Holl
                                            Authorized Signatory






                                         Robert Day

                                         By: /s/ Philip K. Holl
                                             --------------------
                                             Philip K. Holl
                                             Under Power of Attorney dated
                                             January 19, 2000, on File with
                                             Schedule 13G Amendment Number 2
                                             for J. Baker, Inc. dated
                                             January 20, 2000.







<PAGE>

                                   EXHIBIT A
                RELEVANT SUBSIDIARIES OF PARENT HOLDING COMPANY

PART A: TCW ENTITIES

PARENT HOLDING COMPANY:

     The TCW Group, Inc.

     Robert Day (an individual who may be deemed to control The TCW Group,
     Inc.)


RELEVANT SUBSIDIARIES THAT ARE PERSONS DESCRIBED IN RULE 13d-1(b):

     (i)  Trust Company of the West, a California corporation and a bank as
defined in Section 3(a)(6) of the Securities Exchange Act of 1934.
     
     (ii) TCW Asset Management Company, a California corporation and an
Investment Adviser registered under Section 203 of the Investment Advisers Act
of 1940.

Note:     No Common Stock of Itron, Inc. is held directly by The TCW Group,
Inc. Other than the indirect holdings of The TCW Group, Inc., no Common Stock
of Itron, Inc. is held directly or indirectly by Robert Day, an individual who
may be deemed to control The TCW Group, Inc.

PART B: NON TCW ENTITIES

PARENT HOLDING COMPANY:

     Robert Day (an individual who may be deemed to control the entities
     described below which are not subsidiaries of The TCW Group, Inc.)

RELEVANT SUBSIDIARIES THAT ARE PERSONS DESCRIBED IN RULE 13d-1(b):

     Oakmont Corporation, a California corporation and an Investment Adviser
     registered under Section 203 of the Investment Advisers Act of 1940.



                                      A-1






<PAGE>

                                   EXHIBIT B
                            JOINT FILING AGREEMENT


     The undersigned acknowledge and agree that the foregoing statement on
Schedule 13G is filed on behalf of each of the undersigned and that all
subsequent amendments to this statement on Schedule 13G shall be filed on
behalf of each of the undersigned without the necessity of filing additional
joint acquisition statements.  The undersigned acknowledge that each shall be
responsible for the timely filing of such amendments, and for the completeness
and accuracy of the information concerning him or it contained therein, but
shall not be responsible for the completeness and accuracy of the information
concerning the other, except to the extent that he or it knows or has reason
to believe that such information is accurate.


Dated this 11th day of February, 2000.


                                        The TCW Group, Inc.

                                        By: /s/ Philip K. Holl
                                            --------------------
                                            Philip K. Holl
                                            Authorized Signatory






                                         Robert Day

                                         By: /s/ Philip K. Holl
                                             --------------------
                                             Philip K. Holl
                                             Under Power of Attorney dated
                                             January 19, 2000, on File with
                                             Schedule 13G Amendment Number 2
                                             for J. Baker, Inc. dated
                                             January 20, 2000.



                                      B-1