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                                                                   EXHIBIT 10.13

                           THIRD AMENDMENT TO LEASE
                           ------------------------

     THIS THIRD AMENDMENT TO LEASE (this "Third Amendment") dated as of this
16/th/ day of February, 1999, is entered into by and between ZORO, LLC, a
California limited liability company ("Landlord"), and FOCAL COMMUNICATIONS
CORPORATION, a Delaware corporation ("Tenant").

     The Parties enter into this Third Amendment on the basis of the following
facts, understandings and intentions:


                                   RECITALS


     A.   Capitalized terms used in this Third Amendment without definition
shall have the meanings ascribed to such terms in the Original Lease (as
hereinafter defined).

     B.   Landlord and Tenant entered into that certain Lease dated January 26,
1998 ("Original Lease"), whereby Landlord leased to Tenant and Tenant leased
from Landlord certain premises more particularly described therein (the
"Original Premises"), located on the second floor of the building owned by
Landlord at 650 Townsend Street, San Francisco, California (the "Building").

     C.   Landlord and Tenant subsequently entered into that certain First
Amendment to Lease dated March 3, 1998 and that certain Second Amendment to
Lease dated June 16, 1998. (The Original Lease and the First and Second
Amendments are collectively referred to herein as the "Lease").

     D.   Tenant desires to lease additional space in the Building (the
"Expansion Premises"). The Expansion Premises consist of approximately 2,654
Adjusted Rentable Square Feet as identified on the floor plan attached as
Exhibit A-1 hereto.

     E.   Landlord and Tenant also desire to amend the Lease to reflect an
increase in the Adjusted Rentable Square Footage of the Building of which the
Original and Expansion Premises are a part from 666,711 Adjusted Rentable Square
Feet to 670,604 Adjusted Rentable Square Feet after measurement by Landlord's
consultant in or about July, 1998.

     F.   Landlord and Tenant desire to amend the Lease to provide for the
Expansion Premises, to reflect the correct total Adjusted Rentable Square Feet
for the entire Building, to modify the Base Rent, and certain other matters as
provided herein.

                                       1.
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          NOW, THEREFORE, in consideration of the agreements herein contained,
and for other good and valuable consideration, the receipt and adequacy of which
is hereby acknowledged, Landlord and Tenant agree as follows:



                                  AGREEMENTS



1.   THE EXPANSION PREMISES.

     1.1  Delivery of Expansion Premises. Landlord shall deliver the Expansion
          ------------------------------                                      
Premises to Tenant within thirty (30) days after execution of this Third
Amendment ("The Expansion Premises Delivery Date").

     1.2  AS-IS Condition. The Expansion Premises is delivered to Tenant in an
          ---------------     
"AS-IS" condition and without any obligation on the part of Landlord to perform
any improvements. Except as expressly stated in this Third Amendment, Landlord
disclaims any warranties with respect to the condition, suitability, fitness or
merchantability of the Expansion Premises, and Tenant agrees that it is relying
upon its own examination of the condition of the Expansion Premises.

     1.3  Tenant's Improvements.
          --------------------- 

          (a)    After the Expansion Premises Delivery Date, Tenant agrees to
perform, at its sole cost and expense, the following construction work
(hereafter "Tenant's Improvements"): (i) Install building standard carpet and
paint; (ii) remove existing demising wall within the Expansion Premises; and
(iii) Tenant shall connect all electrical circuitry for the Expansion Premises
to the electric meter presently servicing the Original Premises. Upon expiration
of the Expansion Premises Term, at Landlord's option, Landlord may require
Tenant to reconnect the electrical service for the Expansion Premises to the
Building's electric meter.

          (b)    Tenant's Improvements shall be constructed with the Landlord's
contractor and consultants and shall be directly supervised by Landlord's
Project Manager. In consideration of the supervisory, logistical and review work
to be performed by Landlord in connection with Tenant's Improvements, Tenant
agrees to pay Landlord a "Coordination Fee" equal to $2,654.00. Such
coordination fee shall be paid within ten (10) days of billing by Landlord.

          (c)    Tenant shall contract directly with Landlord's contractor and
consultants to construct Tenant's Improvement.

                                       2.
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          (d)    Tenant acknowledges that Landlord has established procedures
for relations with the Building and Planning Departments of the City and County
of San Francisco and that Tenant, Tenant's representatives, architects, or
agents shall not contact any representatives of the City and County of San
Francisco without the presence of Landlord's representative to assure
consistency of treatment of the Building and its tenants by such governmental
agencies. Any such contact by Tenant's representatives in contravention of this
provision which causes an alteration in governmental treatment of the Building
which results in additional costs to the Building or any tenant therein, shall
be borne by the Tenant.

          (e)    Subject to the provisions of paragraph 1.3(d), Tenant shall be
solely responsible for obtaining all necessary governmental approvals and
permits (including but not limited to the approval of the San Francisco City
Planning Department) required to commence and complete the Tenant's Improvements
after obtaining the prior approval of Landlord before making any submittal to
any governmental agency for permit, which approval of Landlord shall not be
unreasonably withheld; and immediately upon receipt thereof, Tenant shall
deliver copies of all such approvals and permits to Landlord.

          (f)    Except as expressly set forth to the contrary in the Lease with
respect to Code Compliance, it shall be Tenant's sole responsibility to satisfy
all applicable building code requirements and governmental rules and regulations
concerning the design and construction of the Tenant's Improvements. Tenant
acknowledges that applying for necessary governmental approvals and permits may,
as a consequence, require Tenant to construct improvements beyond those
contemplated in Section 1.3(a). Tenant, at its sole cost and expense, shall
construct any and all improvements, charges and alterations required by
governmental authorities by reason of applying for governmental permits or
actually constructing Tenant's Improvements including, without limitation, Life
Safety review compliance, ADA, etc.

    1.4   Adjusted Rentable Floor Area. Within thirty (30) days of the Expansion
          ----------------------------                                          
Premises Delivery Date, Landlord's Contractor or Architect, at Tenant's expense,
shall measure the Expansion Premises to determine the exact Adjusted Rentable
Area of the Expansion Premises. Such determination shall be binding upon
Landlord and Tenant and the parties shall execute a letter agreement confirming
the Adjusted Rentable Area. Base Rent and other charges shall be proportionately
adjusted to reflect any change in the Adjusted Rentable Floor area from 2,654
square feet.

2.   LEASE AMENDMENT.

     Effective upon the Expansion Premises Delivery Date, the Lease shall,
without further documentation, be amended as follows:

     2.1  Expansion Premises Term. The term of the Lease for the Expansion Space
          -----------------------                                               
only shall commence on the Expansion Premises Delivery Date and shall expire on
----
the last day of the month immediately following the expiration of five (5)
calendar years ("the Expansion Premises Term"). There are no option(s) to renew
the term.

                                       3.
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     2.2  Premises. Effective upon the Expansion Premises Delivery Date, the
          --------
Expansion Premises shall be added to the description and size of the Original
Premises. Subject to adjustment as provided in Section 1.4 of this Third
Amendment, the total size of the Original Premises and Expansion Premises is
20,151 square feet of Adjusted Rentable Area. Upon expiration of the Expansion
Premises Term, the size of the Premises shall be reduced to the Adjusted
Rentable Area for the Original Premises.

     2.3  Base Rent. Base Rent payable by Tenant during the Expansion Premises
          ---------      
Term shall be increased by Six Thousand Six Hundred Thirty Five Dollars ($6,635)
per month, Seventy Nine Thousand Six Hundred Twenty Dollars ($79,620) per year
($30 per square foot per year of Adjusted Rental Area applicable to the
Expansion Premises only).

     2.4  Additional Charges. Effective upon the Expansion Premises Delivery
          ------------------                                                
Date and continuing until the expiration of the Expansion Premises Term, all
provisions of the Lease requiring payment by Tenant of a pro rata share which is
based upon the Adjusted Rentable Area of Tenant's premises. (Basic Lease
Information, Item N) shall be based upon the aggregate total of Adjusted
Rentable Area contained in the Expansion Premises and the Original Premises.
Subject to adjustment as provided in Section 1.4, Tenant's aggregate Percentage
Share during the Expansion Premises Term is 3.01%.

     2.5  Security Deposit. Concurrently with the execution of this Third
          ----------------
Amendment, the Security Deposit referenced as Item M, Basic Information of the
Lease, shall be increased by payment to Landlord of Six Thousand Six Hundred
Thirty Five Dollars ($6,635). Subject to the terms of Sections 3.5(b), (c) and
(d), Landlord shall refund to the increased Security Deposit amount of $6,635 to
Tenant upon expiration of the Expansion Premises Term.

     2.6  Building Construction. Tenant acknowledges that the Building is a
          ---------------------                                            
redevelopment project and is currently undergoing construction, during normal
business hours. Such construction shall, from time to time, and for an
indefinite period, create higher than normal noise levels and make it difficult
for Landlord to keep the Building in a clean condition.

     2.7  Building Adjusted Rentable Area. The Adjusted Rentable Area of the
          -------------------------------                                   
entire Building in Item N, Basic Information of the Lease is amended to read
670,604 square feet in lieu of 666,7111 square feet.

3.   DEFINED TERMS.

     As used herein, capitalized terms shall have the meaning ascribed to them
in the Lease, unless expressly defined herein.

4.   LEASE IN FULL FORCE AND EFFECT.

     Except as hereby modified, the Lease remains unchanged and in full force
and effect.

                                       4.
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5.   SUCCESORS AND ASSIGNS.

     This Third Amendment shall bind and inure to the benefit of the parties
hereto and its respective successors and assigns.

6.   BROKERS.

     Tenant represents and warrants to Landlord that Tenant has had no dealings
with any broker, finder or similar person who is or might be entitled to a
commission or other fee in connection with this Third Amendment. Tenant shall
protect, defend and indemnify Landlord against, and hold Landlord harmless from,
any and all claims, demands, liability and costs (including reasonable
attorneys' fees), of any person, other than Landlord's Broker, who claims to
have dealt with Tenant in connection with the transaction contemplated by this
Third Amendment. Landlord shall protect, defend and indemnify Tenant against,
and hold Tenant harmless from, any and all claims, demands, liability and costs
(including reasonable attorneys' fees), of any person who claims to have dealt
with Landlord in connection with the transaction contemplated by this Third
Amendment.

7.   NO DEFAULT.

     Tenant represents and warrants that there are no defaults of Landlord under
the Lease or any existing conditions, which upon the giving of notice or the
lapse of time, or both, would constitute default under the Lease.

8.   OTHER INSTRUMENTS.

     Landlord and Tenant agree that they shall execute such other and further
documents as are necessary or convenient to effect the objectives of this Third
Amendment.

     IN WITNESS WHEREOF, the parties have executed this Amendment as of the date
and year above written.

                         "LANDLORD"     
                         ZORO, LLC, a California limited liability company 
                         

                         By:  /s/ Martin Zankel
                              --------------------------------------------- 
                         Its:     Managing Member
                              ---------------------------------------------


                         "TENANT"
                         FOCAL COMMUNICATIONS CORPORATION, a
                         Delaware corporation

                         By:  /s/ John Barnicle
                              ---------------------------------------------
                         Its:  E.V.P C.O.O
                               --------------------------------------------

                                       5.
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                                  EXHIBIT A-I

                    [PLAN OF TOWNSEND CENTER APPEARS HERE]

                                       6.