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                                                                     Exhibit 4.3


                                                                  Execution Copy
                                                                  --------------


NEITHER THE SECURITY EVIDENCED BY THIS WARRANT NOR THE SECURITIES ISSUABLE UPON
EXERCISE OF THIS WARRANT HAVE BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933,
AS AMENDED.  SUCH SECURITIES MAY NOT BE SOLD OR TRANSFERRED IN THE ABSENCE OF
SUCH REGISTRATION UNLESS THE COMPANY RECEIVES AN OPINION OF LEGAL COUNSEL FOR
THE HOLDER OF SAID SECURITIES THAT IS REASONABLY ACCEPTABLE TO THE COMPANY,
STATING THAT SUCH SALE OR TRANSFER IS EXEMPT FROM THE REGISTRATION REQUIREMENTS
OF THE SAID ACT OR THE COMPANY OTHERWISE SATISFIES ITSELF THAT SUCH TRANSACTION
IS EXEMPT FROM REGISTRATION.

                              COMMON STOCK WARRANT
                                       OF
                              CAIS INTERNET, INC.

     THIS CERTIFIES THAT, subject to the terms and conditions of this Warrant,
for the consideration of $1.00, the receipt of which is hereby acknowledged,
U.S. Telesource, Inc., a Delaware corporation, or its successors and assigns
(the "holder"), is entitled to purchase, at any time and from time to time on or
after the date hereof, shares of Common Stock, par value $.01 per share (the
"Common Stock"), of CAIS Internet, Inc., a Delaware corporation (the "Company"),
from the Company in such number and at such price as determined in accordance
with this Warrant.

     Upon delivery of this Warrant (with the notice of exercise in the form
attached hereto as Exhibit A), together with payment of the Warrant price (as
defined below) for the shares of Common Stock to be issued, which payment may be
made by converting this warrant, or any portion thereof, pursuant to section 5
below ("Warrant Conversion"), at the principal office of the Company or at such
other office or agency as the Company may designate by notice in writing to the
Holder hereof, the Holder shall be entitled to receive a certificate or
certificates for the shares of Common Stock so purchased.  All shares of Common
Stock that may be issued upon the exercise of this Warrant will, upon issuance,
be fully paid and nonassessable and free from all taxes, liens and charges with
respect thereto.

     This Warrant is subject to the following terms and conditions:

     1.  Term of Warrant. This Warrant may be exercised in whole or in part, at
any time and from time to time on or after the date hereof; provided, however,
                                                            --------  -------
that this Warrant shall expire to the extent then unexercised as of 5:00 p.m.,
eastern time on October 28, 2002.

     2.  Number of Warrant Shares. Subject to adjustment from time to time
pursuant to Section 4 hereof, the Holder may exercise this Warrant with respect
to 500,000 shares of Common Stock (or other securities issuable in the event of
a reclassification, change, merger or consolidation as set forth in Section 4(a)
hereof) (the "Shares").

     3.  Warrant Price. The exercise price of this Warrant (the "Warrant Price")
shall equal $12.00 per share, subject to adjustment from time to time pursuant
to Section 4 hereof.
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     4.  Adjustment of Number of Shares and Warrant Price. The number and kind
of Shares purchasable upon the exercise of the Warrant and the Warrant Price
shall be subject to adjustment from time to time in accordance with the
following provisions:

         (a)  Special Definitions. For purposes of this Section 4, the following
definitions shall apply:

              (i)   "Option" shall mean rights, options or warrants to subscribe
     for, purchase or otherwise acquire Common Stock or Convertible Securities,
     excluding rights or options granted to employees, vendors, officers,
     directors and executives of, and consultants or shareholders to, the
     Company in an amount not exceeding the number of Reserved Employee Shares.

              (ii)  "Convertible Securities" shall mean any evidences of
     indebtedness, shares or other securities directly or indirectly convertible
     into or exchangeable for Common Stock.

              (iii) "Additional Shares of Common Stock" shall mean all shares of
     Common Stock issued (or, pursuant to Section 4(c) below, deemed to be
     issued) by the Company at any time while this Warrant remains outstanding
     and unexpired, other than Reserved Employee Shares and other than shares of
     Common Stock issued or issuable:

                    (i)   upon the exercise of Options;

                    (ii)  as a dividend or distribution on Series C Preferred
              Stock, par value $.01 per share ("Series C Preferred Stock") or
              upon conversion of shares of Series C Preferred Stock;

                    (iii) pursuant to this Warrant and warrants issued by the
              Company pursuant to or in connection with (a) the Warrant
              Agreement dated as of September 4, 1998 among the Company,
              Cleartel Communications, Inc., CAIS, Inc. and ING (U.S.) Capital
              Corporation, Inc. (the "ING Warrant Agreement"), (b) the Series A
              Preferred Stock and Warrant Purchase Agreement dated as of
              February 19, 1999 among the Company and the several purchasers set
              forth therein; and (c) the Warrant to Purchase Common Stock
              granted to Hilton Hotels Corporation;

                    (iv) to a corporation, partnership or other entity with
              which the Company is seeking to establish a partnership, joint
              venture or other business relationship when the total number of
              shares of Common Stock so issuable or issued does not exceed
              1,000,000 shares (as appropriately adjusted for any stock
              dividends, combinations, splits or the like with respect to shares
              of Common Stock), provided the Company receives at least 95% of
              Fair Market Value for such shares;

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                    (v)  in connection with any high-yield debt financing
              undertaken by the Company, not to exceed 2,000,000 shares of
              Common Stock in the aggregate;

                    (vi) pursuant to the Agreement and Plan of Merger among the
              Company, Business Anywhere USA, Inc., CIBA Merger Corp., Kim Kao,
              and Amy Hsiao dated September 7, 1999, including without
              limitation, the conversion of Business Anywhere options into
              options to acquire Common Stock and the issuance of shares of
              Common Stock upon the exercise thereof, not to exceed 288,371
              shares of Common Stock in the aggregate;

                    (vii) in connection with the acquisition by the Company of
              the securities or assets of another corporation, partnership or
              other entity, provided the Company receives at least 95% of Fair
              Market Value for such shares; and

                    (viii) pursuant to the Agreement and Plan of Merger among
              the Company, CIAM Corp. and Atcom, Inc. dated August 4, 1999,
              including without limitation, the conversion of Atcom options into
              options to acquire Common Stock as described therein, the issuance
              of shares of Common Stock upon the exercise thereof and the
              issuance of Common Stock constituting "Contingent Consideration"
              as defined therein, not to exceed 2,654,826 shares of Common Stock
              in the aggregate.

              (iv) "Reserved Employee Shares" shall mean shares of Common Stock
     issued to employees, officers, directors, shareholders and executives of,
     and consultants or vendors to, the Company of up to: (i) 5,000,000 shares
     (as appropriately adjusted for any stock dividends, combinations, splits or
     the like with respect to shares of Common Stock), plus such additional
     number of shares of Common Stock issued or deemed issued for like purposes
     as shall be approved by the Holder; plus (ii) shares reserved, as of the
     date hereof, for issuance upon the exercise of outstanding Options to
     purchase up to 2,604,495 shares of Common Stock presently held by five
     management employees of the Company. Such Reserved Employee Shares shall be
     issued, at any time, and from time to time, under such arrangements,
     contracts or plans as are recommended by the Company's management and
     approved by the Board.

              (v)  "Rights to Acquire Common Stock" (or "Rights") shall mean all
     rights issued by the Company to acquire Common Stock whether by exercise of
     a warrant, option or similar call, or conversion of any existing
     instruments, in either case for consideration fixed, in amount or by
     formula, as of the date of issuance.

              (vi) "Fair Market Value" of any property shall mean the fair
     market value thereof as determined in good faith by the Board of Directors
     of the Company (the "Board"); provided, however, that the value of any
                                   --------  -------
     securities will be determined as follows:

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                    (i)  Securities not subject to investment letter or other
     similar restrictions on free marketability covered by (ii) below:

                         (A) If traded on a securities exchange or through the
     Nasdaq National Market, the value shall be deemed to be the average of the
     closing prices of the securities on such quotation system over the thirty
     (30) day period ending three (3) days prior to the closing;

                         (B) If actively traded over-the-counter, the value
     shall be deemed to be the average of the closing bid or sale prices
     (whichever is applicable) over the thirty (30) day period ending three (3)
     days prior to the closing; and

                         (C) If there is no active public market, the value
     shall be the fair market value thereof, as mutually determined by the Board
     and the Holder.

                    (ii) The method of valuation of securities subject to
     investment letter or other restrictions on free marketability (other than
     restrictions arising solely by virtue of a stockholder's status as an
     affiliate or former affiliate) shall be to make an appropriate discount
     from the market value determined as above in (i)(A), (B) or (C) to reflect
     the approximate fair market value thereof, as mutually determined by the
     Board and the Holder.

        (b) No Adjustment of Warrant Price. The number of Shares purchasable
upon the exercise of the Warrant shall not be adjusted, by adjustment in the
Warrant Price thereof, unless the Fair Market Value of the consideration per
share (determined pursuant to Section 4(e) below) received by the Company for an
Additional Share of Common Stock issued or deemed to be issued by the Company is
less than the greater of 95% of the Fair Market Value per share of the Common
Stock or the applicable Warrant Price in effect on the date of, and immediately
prior to, the issue of such additional shares, or if prior to such issuance, the
Company receives written notice from the Holder, agreeing that no such
adjustment shall be made as the result of the issuance of Additional Shares of
Common Stock.

        (c) Issue of Securities Deemed Issue of Additional Shares of Common
Stock. If the Company at any time or from time to time while this Warrant
remains outstanding and unexpired shall issue any Options or Convertible
Securities or Rights to Acquire Common Stock, then the maximum number of shares
of Common Stock (as set forth in the instrument relating thereto without regard
to any provision contained therein for a subsequent adjustment of such number)
issuable upon the exercise of such Options, Rights to Acquire Common Stock or,
in the case of Convertible Securities, the conversion or exchange of such
Convertible Securities, shall be deemed to be Additional Shares of Common Stock
issued as of the time of such issue; provided, however, that Additional Shares
of Common Stock shall not be deemed to have been issued unless the Fair Market
Value of the consideration per share (determined pursuant to Section 4(e)
hereof) received by the Company for such Additional Shares of Common Stock

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would be less than the greater of 95% of the Fair Market Value per share of
Common Stock or the applicable Warrant Price in effect on the date of and
immediately prior to such issue, or such record date, as the case may be, and
provided, further, that in any such case:

              (i)  No further adjustment in the Warrant Price shall be made upon
     the subsequent issue of shares of Common Stock upon the exercise of such
     Options, Rights or conversion or exchange of such Convertible Securities;

              (ii) Upon the expiration or termination of any unexercised Option,
     Right or Convertible Security, the Warrant Price shall be adjusted
     immediately to reflect the applicable Warrant Price which would have been
     in effect had such Option, Right or Convertible Security (to the extent
     outstanding immediately prior to such expiration or termination) never been
     issued; and

              (iii) In the event of any change in the number of shares of Common
     Stock issuable upon the exercise, conversion or exchange of any Option,
     Right or Convertible Security, including, but not limited to, a change
     resulting from the anti-dilution provisions thereof, the Warrant Price then
     in effect shall forthwith be readjusted to such Warrant Price as would have
     been obtained had the Warrant Price adjustment that was originally made
     upon the issuance of such Option, Right or Convertible Security, which were
     not exercised or converted prior to such change, been made upon the basis
     of such change, but no further adjustment shall be made for the actual
     issuance of Common Stock upon the exercise or conversion of any such
     Option, Right or Convertible Security.

        (d)  Adjustment of Warrant Price upon Issuance of Additional Shares of
Common Stock. If the Company shall at any time while this Warrant is outstanding
and unexpired issue Additional Shares of Common Stock (including Additional
Shares of Common Stock deemed to be issued pursuant to Section 4(c), but
excluding shares issued as a dividend or distribution as provided in Section
4(g) or upon a stock split or combination as provided in Section 4(f)), without
consideration, or for a consideration per share less than the greater of 95% of
the Fair Market Value per share of Common Stock or the applicable Warrant Price
in effect on the date of and immediately prior to such issue, or without the
requisite notice contemplated by Section 4(b) hereof, then and in such event,
such Warrant Price shall be reduced, concurrently with such issue, to a price
(calculated to the nearest cent) determined by multiplying such Warrant Price by
a fraction, the numerator of which shall be the number of shares of Common Stock
outstanding immediately prior to such issue plus the number of shares of Common
Stock which the aggregate consideration received by the Company for the total
number of Additional Shares of Common Stock so issued would purchase at the
greater of 95% of the Fair Market Value per share of Common Stock or such
Warrant Price; and the denominator of which shall be the number of shares of
Common Stock outstanding immediately prior to such issue plus the number of such
Additional Shares of Common Stock so issued.

Notwithstanding the foregoing, the applicable Warrant Price shall not be reduced
if the amount of such reduction would be an amount less than $.03, but any such
amount shall be carried forward and reduction with respect thereto made at the
time of and together with any subsequent

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reduction which, together with such amount and any other amount or amounts so
carried forward, shall aggregate $.03 or more.

        (e)  Determination of Consideration. For purposes of this Section 4, the
Fair Market Value of the consideration received by the Company for the issue of
any Additional Shares of Common Stock shall be computed as follows:

             (i)  Cash and Property.  Such consideration shall:
                  -----------------
                  (i)  insofar as it consists of cash, be computed at the
             aggregate of cash received by the Company, excluding amounts paid
             or payable for accrued interest or accrued dividends;

                  (ii) insofar as it consists of property other than cash, be
             computed at the Fair Market Value thereof at the time of such
             issue, as determined in good faith by the Board; and

                  (iii)  in the event Additional Shares of Common Stock are
             issued together with other shares or securities or other assets of
             the Company for consideration which covers both, be the proportion
             of such consideration so received, computed as provided in clauses
             (i) and (ii) above, as determined in good faith by the Board.

             (ii) Options, Rights and Convertible Securities. The consideration
                  ------------------------------------------
     per share received by the Company for Additional Shares of Common Stock
     deemed to have been issued pursuant to Section 4(c), relating to Options,
     Rights and Convertible Securities, shall be determined by dividing

                  (i)  the total amount, if any, received or receivable by the
             Company as consideration for the issue of such Options, Rights or
             Convertible Securities, plus the minimum aggregate amount of
             additional consideration (as set forth in the instruments relating
             thereto, without regard to any provision contained therein for a
             subsequent adjustment of such consideration) payable to the Company
             upon the exercise of such Options, Rights or the conversion or
             exchange of such Convertible Securities, by

                  (ii) the maximum number of shares of Common Stock (as set
             forth in the instruments relating thereto, without regard to any
             provision contained therein for a subsequent adjustment of such
             number) issuable upon the exercise of such Options, Rights or the
             conversion or exchange of such Convertible Securities.

        (f)  Adjustment for Stock Splits and Combinations. If at any time or
from time to time while this Warrant remains outstanding and unexpired, the
Company shall effect a subdivision of the outstanding Common Stock, the Warrant
Price then in effect immediately

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before that subdivision shall be proportionately decreased. If at any time or
from time to time while this Warrant remains outstanding and unexpired, the
Company shall combine the outstanding shares of Common Stock, the Warrant Price
then in effect immediately before the combination shall be proportionately
increased. Any adjustment under this paragraph shall become effective at the
close of business on the date that the subdivision or combination becomes
effective.

        (g)  Adjustment for Certain Dividends and Distributions. In the event
that, while this Warrant remains outstanding and unexpired, the Company at any
time or from time to time shall make or issue a dividend or other distribution
payable in Additional Shares of Common Stock, then and in each such event the
Warrant Price shall be decreased as of the time of such issuance, by multiplying
the Warrant Price by a fraction, the numerator of which shall be the total
number of shares of Common Stock issued and outstanding immediately prior to the
time of such issuance, and the denominator of which shall be the total number of
shares of Common Stock issued and outstanding immediately prior to the time of
such issuance plus the number of shares of Common Stock issuable in payment of
such dividend or distribution.

        (h)  Adjustments for Other Dividends and Distributions. In the event
that, while this Warrant remains outstanding and unexpired, the Company at any
time, or from time to time shall make or issue, a dividend or other distribution
payable in securities of the Company other than shares of Common Stock, then and
in each such event provision shall be made so that the Holder shall receive upon
the exercise of the Warrant in addition to the number of Shares receivable
thereupon, the amount of securities of the Company that the Holder would have
received had the Warrant been exercised with respect to the Shares on the date
of such event and had thereafter, during the period from the date of such event
to and including the date on which the Warrant was exercised, retained such
securities receivable by the Holder as aforesaid during such period given
application to all adjustments called for during such period.

        (i)  Reclassification, Consolidation or Merger. In case of any capital
reorganization, reclassification or change of outstanding securities of the
class issuable upon exercise of the Warrant (other than as a result of a
subdivision, split, combination or stock dividend), or in case of any
consolidation or merger of the Company with or into another entity, the Company,
or such successor entity, as the case may be, shall execute a new Warrant, with
substantially the same terms as this Warrant, or amend this Warrant, to provide
that the Holder shall have the right to exercise such new Warrant or amended
Warrant and procure upon such exercise in lieu of the Common Stock theretofore
issuable upon exercise of this Warrant the kind and amount of shares of stock,
other securities, money and/or property receivable upon such reorganization,
reclassification, change, consolidation or merger by the Holder as if this
Warrant had been fully exercised immediately prior to such event. Any such new
Warrant shall provide for adjustments which shall be as nearly equivalent as may
be practicable to the adjustments provided for in this Section 4. The provisions
of this subsection (a) shall similarly apply to successive reorganizations,
reclassifications, changes, consolidations and mergers.

        (j)  Certificate as to Adjustments. Upon the occurrence of each
adjustment or readjustment of the Warrant Price pursuant to this Section 4, the
Company at its expense shall promptly compute such adjustment or readjustment in
accordance with the terms hereof and

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furnish to the Holder a certificate setting forth such adjustment or
readjustment and showing in detail the facts upon which such adjustment or
readjustment is based and shall file a copy of such certificate with its
corporate records. The Company shall, upon the reasonable written request of the
Holder, furnish or cause to be furnished to the Holder a similar certificate
setting forth (i) such adjustments and readjustments, (ii) the Warrant Price
then in effect, and (iii) the number of Shares and the amount, if any, of other
property which then would be received upon the exercise of the Warrant.

        5.  Payment by Warrant Conversion. The Holder may exercise the purchase
right represented by this Warrant with respect to a particular number of Shares
subject to this Warrant ("Converted Warrant Shares") and elect to pay for a
number of such Converted Warrant Shares through Warrant Conversion by specifying
such election in the Notice of Exercise attached hereto as Exhibit A. In such
event, the Company shall deliver to the Holder (without payment by the Holder of
any Warrant Price or any cash or other consideration) that number of Shares
equal to the quotient obtained by dividing (x) the value of this Warrant (or the
specified portion hereof) on the date of exercise, which value shall be
determined by subtracting (A) the aggregate Warrant Price of the Converted
Warrant Shares immediately prior to the exercise of the Warrant from (B) the
aggregate fair market value of the Converted Warrant Shares issuable upon
exercise of this Warrant (or the specified portion hereof) on the date of
exercise, by (y) the fair market value of one Share on the date of exercise. For
purposes of this Section 5, fair market value of a Share as of a particular date
shall be the closing price on the business day immediately prior to the exercise
of the applicable Warrant.

        6.  Notices. Upon any adjustment of the Warrant Price and any increase
or decrease in the number of Shares purchasable upon the exercise of this
Warrant, then, and in each such case, the Company, within 30 days thereafter,
shall give written notice thereof to the registered holder of this Warrant (the
"Notice"). The Notice shall be mailed to the address of such holder as shown on
the books of the Company; and shall state the Warrant Price as adjusted and the
increased or decreased number of shares purchasable upon the exercise of this
Warrant, setting forth in reasonable detail the method of calculation of each.

        7.  Transfer and Exchange of the Warrant and Shares. When this Warrant
or Shares are presented to the Company with a request:

            (a)  to register their transfer; or

            (b)  to exchange such Warrant for an equal number of warrants of
other authorized denominations, the Company shall register the transfer or make
the exchange as requested if the following requirements are met:

            (x) the Warrant shall be duly endorsed or accompanied by a written
instruction of transfer in form satisfactory to the Company, duly executed by
the Holder thereof or by his attorney-in-fact, duly authorized in writing; and

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        (y) in the case of Shares, such request shall be accompanied by the
following additional information and documents (all of which may be submitted by
facsimile), as applicable:

            (i)   if such Shares are being transferred (1) to a "qualified
     institutional buyer" (as defined in Rule 144A) in accordance with Rule 144A
     or (2) pursuant to an exemption from registration in accordance with Rule
     144 (and based on an opinion of counsel if the Company so requests) or (3)
     pursuant to an effective registration statement under the Securities Act, a
     certification to that effect;

            (ii)  if such Shares are being transferred pursuant to an exemption
     from registration in accordance with Rule 904 under the Securities Act (and
     based on an opinion of counsel if the Company so requests), a certification
     to that effect; or

            (iii) if such Shares are being transferred in reliance on another
exemption from the registration requirements of the Securities Act (and based on
an opinion of counsel if the Company so requests), a certification to that
effect.

     8.  Representations and Warranties. The Company represents and warrants to
the Holder as follows:

            (a)  Organization and Powers. The Company (i) is a corporation duly
organized, validly existing and in good standing under the laws of the
jurisdiction of its incorporation; and (ii) has all the requisite power and
authority to carry on its business and to execute, deliver and perform its
obligations under this Warrant.

            (b)  Authorization; No Conflict. The offer and sale of the Warrant
and the Common Stock underlying the Warrant, and the execution, delivery and
performance by the Company of the Warrant have been duly authorized by all
necessary corporate action of the Company and do not and will not (i) contravene
the Company's articles of incorporation or bylaws; (ii) result in a breach or
default under any material instrument, contract or other agreement to which the
Company is a party; or (iii) violate any provision of any law, rule, regulation,
order, judgment, decree or the like binding on or affecting the Company.

            (c)  Binding Obligations. The Warrant constitutes, or will
constitute, a legal, valid and binding obligation of the Company, enforceable
against the Company in accordance with its terms, except as may be limited by
bankruptcy, insolvency, reorganization, moratorium or other similar laws
relating to or affecting creditors' rights generally and general principles of
equity, including, without limitation, concepts of materiality, reasonableness,
good faith and fair dealing and the possible unavailability of specific
performance or injunctive relief, regardless of whether considered in a
proceeding of equity or at law.

            (d)  Shares Duly Issued, Fully Paid and Non-Assessable. The Shares,
when issued upon the exercise of this Warrant pursuant to the terms hereof,
shall be duly issued, fully paid and non-assessable.

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            (e)  No Registration. The offer and sale of the Warrant and the
underlying Shares of Common Stock are exempt from the registration and
prospectus delivery requirements of the Securities Act.

        9.  Registration Rights. The Holder shall have such registration rights
with respect to the Shares as specified in that certain Registration Rights and
Lock-Up Agreement dated as of September 29, 1999, as amended by the First
Amendment to Registration Rights and Lock-Up Agreement dated as of even date
herewith.

        10. Press Releases. The Holder shall consent to the form and content of
all press releases or public announcements that shall be made concerning this
Warrant and the transactions contemplated hereby, and the Company shall not make
any press release or public announcement without the Holder's prior written
consent, which consent shall not be unreasonably withheld.

        11.  Miscellaneous.

             (a)  The terms of this Warrant shall be binding upon and shall
inure to the benefit of any successors or assigns of the Company and of the
holder or holders hereof and of the Common Stock issued or issuable upon the
exercise hereof.

             (b)  No Holder, as such, shall be entitled to vote or receive
dividends or be deemed to be a stockholder of the Company for any purpose, nor
shall anything contained in this Warrant be construed (i) to confer upon the
Holder, as such, any rights of a stockholder of the Company, or any right to
vote, give or withhold consent to any corporate action, receive notice of
meetings, receive dividends or subscription rights, or otherwise, or (ii) as
imposing any obligation on the Holder to purchase any securities or any
liability as a stockholder of the Company, whether such obligation or
liabilities are asserted by the Company or its creditors.

             (c)  Receipt of this Warrant by the Holder hereof shall constitute
acceptance of and agreement to the foregoing terms and conditions.

             (d)  The Company will not, by amendment of its certificate of
incorporation or bylaws or through any other action, avoid or seek to avoid the
observance or performance of any of the terms of this Warrant, but will at all
times in good faith assist in the carrying out of all such terms and in the
taking of all actions as may be necessary or appropriate in order to protect the
rights of the Holder against impairment.

             (e)  Upon receipt of evidence reasonably satisfactory to the
Company of the loss, theft, destruction or mutilation of this Warrant and, in
the case of any such loss, theft or distribution, upon delivery of an indemnity
agreement reasonably satisfactory in form and amount to the Company or, in the
case of any such mutilation, upon surrender and cancellation of such Warrants,
the Company at its expense will execute and deliver, in lieu thereof, a new
Warrant of like date and tenor.

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             (f)  Any provision of this Warrant may be amended, waived or
modified upon the written consent of the Company and the Holder.

             (g)  The Company hereby agrees that at all times there shall be
reserved for issuance and/or deliver upon exercise of this Warrant, free from
preemptive rights, such number of authorized but unissued shares of Common Stock
as from time to time shall be required for issuance or delivery upon exercise of
this Warrant. The Company further agrees that it will promptly to take all
action as may from time to time be required in order to permit the holder hereof
to exercise this Warrant and the Company duly and effectively to issue shares of
Common Stock hereunder.

             (h)  This Warrant shall be governed by and construed in accordance
with the laws of the State of New York without regard to the conflicts of laws
provisions thereof.

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     IN WITNESS WHEREOF, the Company has caused this Warrant to be signed by its
duly authorized officer.

Dated: October 27, 1999

                                          CAIS INTERNET, INC.:

                                          /s/ William M. Caldwell, IV
                                          ------------------------------
                                          William M. Caldwell, IV
                                          President

                                      S-1
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                                   EXHIBIT A
                                   ---------
                               NOTICE OF EXERCISE

TO: CAIS Internet, Inc.

     1.  The undersigned hereby elects to purchase _______________ shares of the
Common Stock of CAIS Internet, Inc. pursuant to the terms of the attached
Warrant, and tenders herewith payment of the purchase price of such shares in
full, together with all applicable transfer taxes, if any.

     2.  The undersigned hereby elects to exercise the purchase right with
respect to ___________ shares of such Common Stock through Warrant Conversion,
as set forth in Section 5 of the attached Warrant.

     3.  Please issue a certificate or certificates representing said shares of
Common Stock in the name of the undersigned or in such other names as is
specified below:

          __________________________________________
          (Name)

          __________________________________________
          __________________________________________
          (Address)

                                    Signature of Holder:

                                    By: _________________________________

                                    Title: ______________________________

                                    Date: _______________________________