SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C.  20549

                                   Form 8-A


               For Registration of Certain Classes of Securities
                    Pursuant to Section 12(b) or (g) of the
                        Securities Exchange Act of 1934



                            Sycamore Networks, Inc.
- --------------------------------------------------------------------------------
            (Exact name of registrant as specified in its charter)

Delaware                                                        04-3410558
- ------------------------------------------     ---------------------------------
(State of incorporation                                       (IRS Employer
or organization)                                            Identification No.)

                   10 Elizabeth Drive, Chelmsford, MA  01824
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           (Address of principal executive offices)      (Zip Code)
                                               
If this form relates to the registration of a     If this form relates to the registration of a
class of securities pursuant to Section 12(b)     class of securities pursuant to Section 12(g)
of the Exchange Act and is effective              of the Exchange Act and is effective
pursuant to General Instruction A.(c),            pursuant to General Instruction A.(d),
please check the following box.   [  ]            please check the following box.   [ x ]



Securities Act registration statement file number to which this form relates:
333-84635
- -----------------
(If applicable)

Securities to be registered pursuant to Section 12(b) of the Act:

   Title of Each Class                            Name of Each Exchange on Which
   to be so Registered                            Each Class is to be Registered
   --------------------                           ------------------------------

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

Securities to be registered pursuant to Section 12(g) of the Act:

                    Common Stock, $.001 par value per share
- --------------------------------------------------------------------------------
                               (Title of Class)


Item 1:  Description of Registrant's Securities to be Registered.
         -------------------------------------------------------

         The description under the heading "Description of Capital Stock"
relating to the Registrant's Common Stock, $.001 par value per share, in the
Prospectus included in the Registrant's Registration Statement on Form S-1 filed
with the Securities and Exchange Commission (the "Registration Statement on Form
S-1") (File No. 333-84635) is incorporated herein by reference.


Item 2:  Exhibits.
         --------

         The following exhibits are filed herewith (or incorporated by reference
as indicated below):

         1.   Certificate of Incorporation of the Registrant, as amended,
              incorporated by reference to Exhibit 3.1 to the Company's
              Registration Statement on Form S-1.

         2.   Form of Amended and Restated Certificate of Incorporation of the
              Registrant, incorporated by reference to Exhibit 3.2 to the
              Company's Registration Statement on Form S-1.

         3.   Bylaws of the Registrant, incorporated by reference to Exhibit 3.3
              to the Company's Registration Statement on Form S-1.

         4.   Form of Amended and Restated Bylaws of the Registrant,
              incorporated by reference to Exhibit 3.4 to the Company's
              Registration Statement on Form S-1.


                                   SIGNATURE


     Pursuant to the requirements of Section 12 of the Securities Exchange Act
of 1934, the Registrant has duly caused this registration statement to be signed
on its behalf by the undersigned, thereto duly authorized.



                         Sycamore Networks, Inc.


                         By:  /s/ Frances M. Jewels
                              ------------------------------------------
                              Frances M. Jewels
                              Chief Financial Officer,
                              Vice President, Finance and Administration


September 8, 1999