Document:

<PAGE>
EXHIBIT 10.4

                                                                  EXECUTION COPY

                     THIRD AMENDMENT TO THE CREDIT AGREEMENT

     THIS THIRD AMENDMENT TO THE CREDIT AGREEMENT, dated as of May 12, 2004
(this "AMENDMENT"), is among CHOICE ONE COMMUNICATIONS INC., a Delaware
corporation (the "GUARANTOR"), the subsidiaries of the Guarantor listed on the
signature pages hereto (each a "BORROWER" and collectively the "BORROWERS"), the
banks, financial institutions and other institutional lenders parties to the
Credit Agreement referred to below (collectively, the "LENDERS"), GENERAL
ELECTRIC CAPITAL CORPORATION, as successor administrative agent (in such
capacity, the "ADMINISTRATIVE AGENT") and syndication agent for the Lenders, and
the other agents signatories hereto.

     PRELIMINARY STATEMENTS:

     (1) The Guarantor, the Borrowers, the Lenders, the Administrative Agent,
and certain other agents have entered into the Third Amended and Restated Credit
Agreement dated as of September 13, 2002 (as amended, supplemented or otherwise
modified through the date hereof, the "CREDIT AGREEMENT"). Capitalized terms not
otherwise defined in this Amendment have the same meanings as specified in the
Credit Agreement.

     (2) The Guarantor and the Borrowers have requested that the Lenders agree
to amend certain provisions of the Credit Agreement as hereinafter set forth.

     (3) The Lenders are willing to grant such request of the Guarantor and the
Borrowers, but only on the terms and conditions stated below, and the Guarantor,
the Borrowers and, subject to such terms and limitations, the Lenders have
agreed to amend certain provisions of the Credit Agreement as hereinafter set
forth.

     SECTION 1. Amendment to Credit Agreement. The Credit Agreement is hereby
amended, effective as of the Effective Date (as defined in section 2 below), as
follows:

     (a) Section 5.3 is amended and restated to read in full as follows:

     "In order to compensate the Agents with respect to the Credit Facility, the
Borrowers agree to pay General Electric Capital Corporation the fees set forth
in the separate fee letter agreement executed by the Company with respect to the
Credit Facility dated April 19, 2004."

     (b) Section 7.1(a)(a) is amended and restated to read in full as follows:

     "Deposit Accounts: Securities Accounts (i) Schedule 7.l(a)(a) sets forth
all of the Deposit Accounts and all "securities accounts" (as defined in the
UCC) of the Company and its Subsidiaries.

<PAGE>

     (ii) Legal, binding and enforceable account control agreements or similar
agreements permitting the Administrative Agent to obtain "control" (as such term
is now or hereafter defined in the UCC) are in effect for each Deposit Account
and "securities account" (as defined in the UCC) of the Company and its
Subsidiaries other than the following accounts (the "EXCLUDED ACCOUNTS") so long
as the balance in each such account is less than $20,000: (1) Merrill Lynch
account number 810-07U38 (stock option clearing account), (2) Merrill Lynch
account number 8l0-07Ul5 (purchase card clearing account and (3) Merrill Lynch
account number 810-07002 (purchase card clearing account)."

     (c) Section 9.12(e) is amended and restated to read in full as follows:

     "Simultaneously with (i) the opening of any Deposit Account or any
"securities account" (as defined in the UCC) by the Company or any of its
Subsidiaries or (ii) the balance in any of the Excluded Accounts (as defined in
Section 7.1(a)(a)) exceeding $20,000, cause to be executed and delivered to the
Administrative Agent in form and substance reasonably satisfactory thereto a
control agreement or similar agreement permitting the Administrative Agent to
obtain "control" (as such term is now or hereafter defined in the UCC) over such
Deposit Account, such "securities account" (as defined in the UCC) or such
Excluded Account, as the case may be.

     (d) Section 10.1(a) is amended and restated to read in full as follows:

     "(a) Minimum Available Cash. (i) At any time prior to and including May 10,
2004, permit Available Cash of the Company and its Subsidiaries to be less than
$10,000,000 or (ii) at any time from and including May 11, 2004 to and including
November 15, 2004, permit Available Cash of the Company and its Subsidiaries to
be less than $6,000,000; it being understood and agreed that, notwithstanding
any provisions contained in Article XII to the contrary, the failure by the
Company and its Subsidiaries to be in compliance with the foregoing amounts of
Available Cash at any time during the foregoing periods shall be an immediate
Event of Default, and no grace period or notice requirement shall be
applicable."

     (e) Section 4.5(c)(vi) is amended by adding the following sentence at the
end thereof: "For purposes of clarity, it is understood and agreed that in the
event that the Borrowers shall fail to pay any of the Obligations when due and
the Obligations have been accelerated pursuant to Section 12.2, or upon the sale
or other disposition of any of the collateral security provided pursuant to the
Security Documents, the provisions of Section 5.5 of this Agreement shall
apply."

     (f) Section 15.11 is amended by adding immediately prior to the period at
the end of the last paragraph thereof the following: "and notwithstanding any
provision of the Loan Documents (including the Intercreditor Agreement) to the
contrary, the aggregate principal amount of Debt outstanding under the Credit
Agreement shall not exceed $398,875,000 (excluding any capitalized interest)
without the prior written consent of the Required Lenders".

     (g) (i) Schedule 7.1(a)(a) is amended and restated to read in full as set
forth in Exhibit A attached hereto and (ii) the list of Schedules in the table
of contents is amended by

                                       2

<PAGE>

adding thereto after the reference to Schedule 7.1(w) the following: "Schedule
7.l(a)(a) - Deposit Accounts; Security Accounts".

     SECTION 2. Conditions of Effectiveness. This Amendment shall become
effective as of the date first above written (the "AMENDMENT EFFECTIVE DATE")
when, and only when, the following conditions precedent shall have been
satisfied:

          (a) The Administrative Agent shall have notified the parties hereto
     that it has received counterparts of (i) this Amendment executed by the
     Guarantor, the Borrowers and the Required Lenders or, as to any of the
     Lenders, advice satisfactory to the Administrative Agent that such Lender
     has executed this Amendment, and (ii) the Consent attached to this
     Amendment executed by each of the Pledgors and each of the Grantors
     designated therein;

          (b) The Borrowers and Guarantor shall have paid in full any and all
     accrued fees and expenses of the Administrative Agent in connection with
     all matters relating to the Credit Agreement (other than the agency fee
     referred to in Section 4(d)(i) below) and all fees and expenses of counsel
     and financial advisors to the Administrative Agent invoiced prior to the
     dated hereof; and

          (c) The Borrowers and the Guarantor shall have provided an additional
     cash retainer to counsel for the Administrative Agent, Shearman & Sterling
     LLP, in such amount as has been previously discussed and agreed to.

     SECTION 3. Representations and Warranties of the Borrowers. The Guarantor
and each Borrower, respectively, represents and warrants as follows:

          (a) This Amendment has been duly executed and delivered by the
     Guarantor and the Borrowers. This Amendment and each of the other Loan
     Documents, as amended hereby, to which the Guarantor or any of the
     Borrowers is a party are legal, valid and binding obligations of such
     party, enforceable against such party in accordance with their respective
     terms.

          (b) (i) The representations and warranties set forth in Article VII of
     the Credit Agreement and in the other Loan Documents are true and correct
     on the Amendment Effective Date as if made on the Amendment Effective Date
     and after giving effect to this Amendment (unless stated to relate solely
     to an earlier date, in which case such representations and warranties were
     true and correct in all material respects as of such earlier date); and
     (ii) after giving effect to this Amendment, no Default has occurred and is
     continuing.

                                       3

<PAGE>

     SECTION 4. Reference to and Effect on the Loan Documents: Etc. (a) On and
after the Effective Date, each reference in the Credit Agreement to "this
Agreement", "hereunder", "hereof" or words of like import referring to the
Credit Agreement, and each reference in the Notes and each of the other Loan
Documents to "the Credit Agreement", "thereunder", "thereof" or words of like
import referring to the Credit Agreement, shall mean and be a reference to the
Credit Agreement, as amended by this Amendment.

     (b) The Credit Agreement, as specifically amended by this Amendment, is and
shall continue to be in full force and effect and is hereby in all respects
ratified and confirmed. Without limiting the generality of the foregoing, the
Security Documents and all of the Collateral described therein do and shall
continue to secure the payment of all Obligations of the Borrowers and the
Guarantor under the Loan Documents, in each case as amended by this Amendment.

     (c) The execution, delivery and effectiveness of this Amendment shall not,
except as expressly provided herein, operate as a waiver of any right, power or
remedy of any Lender or the Administrative Agent under the Loan Documents, nor
constitute a waiver of any provision of the Loan Documents.

     (d) The Borrowers and the Guarantor jointly and severally agree (i) to pay
the agency fee agreed upon in the fee letter referred to in Section 5.3 of the
Credit Agreement, as amended by this Amendment, within 15 days after the date
hereof, (ii). to promptly pay all reasonable expenses of General Electric
Capital Corporation in connection with the preparation, execution and delivery
of this Amendment (as well as all outstanding invoices of professionals retained
by the Administrative Agent) and all matters related thereto and (iii) to pay
within 15 days of receipt thereof all invoices outstanding from time to time of
professionals retained by the Administrative Agent.

     (e) It is understood and agreed that following the consummation of a
consensual restructuring of the Guarantor and its subsidiaries and after the
payment in full of any invoices of counsel to the Administrative Agent in
connection with such restructuring and any post-closing invoices of counsel to
the Administrative Agent in respect of such restructuring, any amounts remaining
on retainer with counsel to the Administrative Agent shall be returned promptly
to the Guarantor.

     SECTION 5. Execution in Counterparts. This Amendment may be executed in any
number of counterparts and by different parties hereto in separate counterparts,
each of which when so executed shall be deemed to be an original and all of
which taken together shall constitute but one and the same agreement. Delivery
of an executed counterpart of a signature page to this Amendment by facsimile
shall be effective as delivery of a manually executed counterpart of this
Amendment.

                                       4

<PAGE>

     SECTION 6. Modifications to this Amendment. Any provision of this Amendment
may be modified with the written consent of the Borrowers, the Guarantor and the
Required Lenders, except for any provision that, pursuant to Section 15.11 of
the Credit Agreement, would require the consent of all the Lenders (in which
case the written consent of all the Lenders shall be required).

     SECTION 7. GOVERNING LAW. THIS AMENDMENT SHALL BE GOVERNED BY, AND
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK

                    [Remainder of page intentionally blank]

                                       5
<PAGE>

                 IN WITNESS WHEREOF, the parties hereto have caused this
Amendment to be executed by their respective officers thereunto duly authorized,
as of the date first above written.

                           BORROWERS:

                           CHOICE ONE COMMUNICATIONS OF NEW YORK INC.
                           CHOICE ONE COMMUNICATIONS OF PENNSYLVANIA INC.
                           CHOICE ONE COMMUNICATIONS OF MASSACHUSETTS INC.
                           CHOICE ONE COMMUNICATIONS INTERNATIONAL INC.
                           CHOICE ONE COMMUNICATIONS OF RHODE ISLAND INC.
                           CHOICE ONE COMMUNICATIONS OF CONNECTICUT INC.
                           CHOICE ONE COMMUNICATIONS OF MAINE INC.
                           CHOICE ONE OF NEW HAMPSHIRE INC.
                           CHOICE ONE COMMUNICATIONS OF OHIO INC.
                           CHOICE ONE COMMUNICATIONS OF VERMONT INC.
                           CHOICE ONE ONLINE INC.
                           CHOICE ONE COMMUNICATIONS OF VIRGINIA INC.
                           CHOICE ONE COMMUNICATIONS SERVICES INC.
                           US XCHANGE INC.
                           US XCHANGE OF INDIANA, L.L.C.
                           US XCHANGE OF ILLINOIS, L.L.C.
                           US XCHANGE OF WISCONSIN, L.L.C.
                           US XCHANGE OF MICHIGAN, L.L.C.

                               By: /s/ Ajay Sabherwal
                                  ---------------------------
                                  Name:  Ajay Sabherwal
                                  Title: CFO
                                         for each of the entities set forth
                                         above

                               GUARANTOR:

                               CHOICE ONE COMMUNICATIONS INC.

                               By: /s/ Ajay Sabherwal
                                  ---------------------------
                                  Name:  Ajay Sabherwal
                                  Title: CFO

<PAGE>

                                GENERAL ELECTRIC CAPITAL CORPORATION,
                                as Administrative Agent, Collateral Agent,
                                Syndication Agent and Lender

                                By: /s/ Robert Stefanowki
                                   ---------------------------
                                   Name: Robert Stefanowki
                                   Title: Duly Authorized Assignee

                                BANK OF AMERICA, N.A., as Lender

                                By: /s/ Sid Bridges
                                   ---------------------------
                                   Name: Sid Bridges
                                   Title: AVP

                                CARGILL FINANCIAL SERVICES
                                INTERNATIONAL INC., as Lender

                                By: /s/ Kelly Schreurs
                                   ---------------------------
                                   Name:  Kelly Schreurs
                                   Title: Controller

                                By: /s/ Mark Guidinger
                                   ---------------------------
                                   Name: Mark Guidinger
                                   Title: Portfolio Manager

                                CREDIT SUISSE FIRST BOSTON, as Lender

                                By: /s/ Joseph Brosnan
                                   ---------------------------
                                   Name: Joseph Brosnan
                                   Title: Vice President

<PAGE>

                                DEUTSCHE BANK TRUST COMPANY AMERICAS,
                                as Lender
                                By: DB Services New Jersey, Inc.

                                By: /s/ John Pineiro
                                   ---------------------------
                                   Name: John Pineiro
                                   Title: Director

                                FERNWOOD ASSOCIATES, L.P., as Lender

                                By: /s/ Thomas P. Borger
                                   ---------------------------
                                   Name: Thomas P. Borger
                                   Title: General Partner

                                MERRILL LYNCH CREDIT PRODUCTS, LLC, as
                                Lender

                                By: /s/ Michael Lee
                                   ---------------------------
                                   Name: Michael Lee
                                   Title: Vice President

                                By:
                                   ---------------------------
                                   Name:
                                   Title:

                                MORGAN STANLEY SENIOR FUNDING, INC.,
                                as Documentation Agent and Lender

                                By: /s/ Ian Sandler
                                   ---------------------------
                                   Name: Ian Sandler
                                   Title: Vice President

<PAGE>

                                QUANTUM PARTNERSHIP, as Lender

                                By: /s/ Richard D. Holahan, Jr.
                                   -----------------------------
                                   Name: Richard D. Holahan, Jr.
                                   Title: Attorney-in-Fact

                                SCOGGIN CAPITAL MANAGEMENT, LP II, as
                                Lender

                                By: /s/ Craig Effron
                                   -----------------------------
                                   Name: Craig Effron
                                   Title: President

                                STRATEGIC VALUE MASTER FUND, LTD, as
                                Lender

                                By: Strategic Value Partners, L.L.C.

                                By: /s/ Vivianne Hernandez
                                   -----------------------------
                                   Name: Vivianne Hernandez
                                   Title: Authorized Signatory

                                TRIAGE CAPITAL MANAGEMENT, LP, as
                                Lender

                                By: /s/ Jerry Frankel
                                   -----------------------------
                                   Name: Jerry Frankel

<PAGE>

                                    CONSENT

                                                        Dated as of May 12, 2004

     Each of the undersigned, as a Grantor and/or Pledgor under the Third
Amended and Restated Security Agreement, dated as of September 13, 2002 (as
amended, supplemented or otherwise modified through the date hereof, the
"SECURITY AGREEMENT" and/or the Third Amended and Restated Pledge Agreement,
dated as of September 13, 2002 (as amended, supplemented or otherwise modified
through the date hereof, the "PLEDGE AGREEMENT"), in each case, in favor of the
Administrative Agent, for its benefit and the benefit of the Lenders parties to
the Credit Agreement referred to in the foregoing Third Amendment (the
"AMENDMENT"), hereby consents to such Amendment and hereby confirms and agrees
that (a) notwithstanding the effectiveness of such Amendment, each of the
Security Agreement and the Pledge Agreement is, and shall continue to be, in
full force and effect and is hereby ratified and confirmed in all respects,
except that, on and after the effectiveness of such Amendment, each reference in
the Security Agreement and the Pledge Agreement to the "Credit Agreement",
"thereunder", "thereof" or words of like import shall mean and be a reference to
the Credit Agreement, as amended by such Amendment, and (b) the Security
Documents to which such Grantor or such Pledgor, as the case may be, is a party
and all of the Collateral described therein do, and shall continue to, secure
the payment of all of the Secured Obligations (in each case, as defined
therein).

                               CHOICE ONE COMMUNICATIONS INC.
                               CHOICE ONE COMMUNICATIONS OF NEW YORK INC.
                               CHOICE ONE COMMUNICATIONS OF PENNSYLVANIA INC.
                               CHOICE ONE COMMUNICATIONS OF MASSACHUSETTS INC.
                               CHOICE ONE COMMUNICATIONS INTERNATIONAL INC.
                               CHOICE ONE COMMUNICATIONS OF RHODE ISLAND INC.
                               CHOICE ONE COMMUNICATIONS OF CONNECTICUT INC.
                               CHOICE ONE COMMUNICATIONS OF MAINE INC.
                               CHOICE ONE OF NEW HAMPSHIRE INC.
                               CHOICE ONE COMMUNICATIONS OF OHIO INC.
                               CHOICE ONE COMMUNICATIONS OF VERMONT INC.
                               CHOICE ONE ONLINE INC.
                               CHOICE ONE COMMUNICATIONS OF VIRGINIA INC.
                               CHOICE ONE COMMUNICATIONS SERVICES INC.
                               US XCHANGE INC.
                               US XCHANGE OF INDIANA, L.L.C.
                               US XCHANGE OF ILLINOIS, L.L.C
                               US XCHANGE OF WISCONSIN, L.L.C.
                               US XCHANGE OF MICHIGAN, L.L.C.

                                By: /s/ Ajay Sabherwal
                                   ---------------------------
                                   Name: Ajay Sabherwal
                                   Tile: CFO<PAGE>

Exhibit 10(m) Third Amendment to Credit Agreement

                       THIRD AMENDMENT TO CREDIT AGREEMENT

            THIS THIRD AMENDMENT TO CREDIT AGREEMENT, dated as of March 30, 2004
(this "Amendment"), is among KEITHLEY INSTRUMENTS, INC., an Ohio corporation
(the "Company"), Subsidiary Borrowers (referred to below and collectively with
the Company, the "Borrowers"), the Lender (as referred to below) and BANK ONE,
NA, a national banking association, having its principal office in Columbus,
Ohio, as Agent for the Lender (in such capacity the "Agent").

                                    RECITALS

            A. The Company, certain Subsidiary Borrowers party thereto, the
Lender party thereto and Agent are parties to a Credit Agreement, dated as of
March 30, 2001, as amended by a First Amendment to Credit Agreement dated as of
August 1, 2002, as amended by a Second Amendment to Credit Agreement dated as of
March 28, 2003 (as now and hereafter amended, the "Credit Agreement"), pursuant
to which the Lender agreed, subject to the terms and conditions thereof, to
extend credit to the Borrowers.

            B. The Borrowers desire to amend the Credit Agreement and the Agent
and the Lender are willing to do so strictly in accordance with the terms
hereof.

                                     TERMS

            In consideration of the premises and of the mutual agreements herein
contained, the parties agree as follows:

                                   ARTICLE 1.
                                   AMENDMENTS

            Upon fulfillment of the conditions set forth in Article 3 hereof,
the Credit Agreement shall be amended as follows:

      1.1 The definition of "Facility Termination Date" in Section 1.1 shall be
amended by deleting the reference therein to "March 30, 2004" and inserting
"March 31, 2005" in place thereof.

                                   ARTICLE 2.
                                 REPRESENTATIONS

            Each Borrower represents and warrants to the Agent and the Lender
that:

      2.1 The execution, delivery and performance of this Amendment is within
its powers, has been duly authorized and is not in contravention with any law,
of the terms of its Articles of Incorporation or By-laws, or any undertaking to
which it is a party or by which it is bound.

      2.2 This Amendment is the legal, valid and binding obligation of the
Borrower enforceable against it in accordance with the terms hereof.
<PAGE>
      2.3 After giving effect to the amendments herein contained, the
representations and warranties contained in Article V of the Credit Agreement
are true on and as of March 30, 2004 with the same force and effect as if made
on and as of March 30, 2004.

      2.4 No Default or Unmatured Default exists or has occurred and is
continuing on the date hereof.

                                   ARTICLE 3.
                           CONDITIONS OF EFFECTIVENESS

            This Amendment shall not become effective until each of the
following has been satisfied:

      3.1 This Amendment shall be signed by each Borrower, the Agent and the
Lender.

      3.2 The Company shall have paid an amendment fee in the amount of $2,500.

                                   ARTICLE 4.
                                 MISCELLANEOUS.

      4.1 References in the Credit Agreement or in any note, certificate,
instrument or other document to the "Credit Agreement" shall be deemed to be
references to the Credit Agreement as amended hereby and as further amended from
time to time.

      4.2 The Company agrees to pay and to save the Agent harmless for the
payment of all costs and expenses arising in connection with this Amendment,
including the reasonable fees of counsel to the Agent in connection with
preparing this Amendment and the related documents.

      4.3 Each Borrower acknowledges and agrees that the Agent and the Lender
have fully performed all of their obligations under all documents executed in
connection with the Credit Agreement and all actions taken by the Agent and the
Lender are reasonable and appropriate under the circumstances and within their
rights under the Credit Agreement and all other documents executed in connection
therewith and otherwise available. Each Borrower represents and warrants that it
is not aware of any claims or causes of action against the Agent or any Lender,
any participant lender or any of their successors or assigns.

      4.4 Except as expressly amended hereby, each Borrower agrees that the
Credit Agreement and all other Loan Documents are ratified and confirmed and
shall remain in full force and effect and that it has no set off, counterclaim
or defense with respect to any of the foregoing. Terms used but not defined
herein shall have the respective meanings ascribed thereto in the Credit
Agreement.

      4.5 This Amendment may be signed upon any number of counterparts with the
same effect as if the signatures thereto and hereto were upon the same
instrument.

                                      -2-
<PAGE>
            IN WITNESS WHEREOF, the parties signing this Amendment have caused
this Amendment to be executed and delivered as of March 30, 2004.

                                     KEITHLEY INSTRUMENTS, INC.

                                     By: /s/ Mark J. Plush
                                         -----------------

                                         Its: Vice President & Chief Financial
                                              Officer

                                     KEITHLEY INSTRUMENTS GmbH

                                     By: /s/ Mark J. Plush
                                         -----------------

                                         Its: Managing Director

                                     KEITHLEY INSTRUMENTS SARL

                                     By: /s/ Mark J. Plush
                                         -----------------

                                         Its: Managing Director

                                     KEITHLEY INSTRUMENTS LTD.

                                     By: /s/ Mark J. Plush
                                         -----------------

                                         Its: Managing Director

                                     KEITHLEY INSTRUMENTS SRL

                                     By: /s/ Mark J. Plush
                                         -----------------

                                         Its: Managing Director

                                     KEITHLEY INSTRUMENTS BV

                                     By: /s/ Joseph P. Keithley
                                         ----------------------

                                         Its: Chairman

                                      -3-
<PAGE>
                                     KEITHLEY INSTRUMENTS SA

                                     By: /s/ Mark J. Plush
                                         -----------------

                                         Its: Managing Director

                                     KEITHLEY INSTRUMENTS KK

                                     By: /s/ Mark J. Plush
                                         -----------------

                                         Its: Managing Director

                                     KEITHLEY INTERNATIONAL
                                     INVESTMENT CORP.

                                     By: /s/ Mark J. Plush
                                         -----------------

                                         Its: Director / Secretary Treasurer

                                     BANK ONE, NA, as Agent, LC Issuer
                                      and Lender

                                     By: /s/ Jim Malz
                                         ------------
                                         Its: SVP / President - NE Ohio Region

                                      -4-

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