<PAGE>
                                                                    Exhibit 4.01

          This Note is a Global Security within the meaning of the Indenture
hereinafter referred to and is registered in the name of the Depository named
below or a nominee of the Depository. This Note is not exchangeable for Notes
registered in the name of a Person other than the Depository or its nominee
except in the limited circumstances described herein and in the Indenture, and
no transfer of this Note (other than a transfer of this Note as a whole by the
Depository to a nominee of the Depository or by a nominee of the Depository to
the Depository or another nominee of the Depository) may be registered except in
the limited circumstances described herein.

          Unless this certificate is presented by an authorized representative
of The Depository Trust Company, a New York corporation (the "Depository"), to
the Company or its agent for registration of transfer, exchange, or payment, and
any certificate issued is registered in the name of Cede & Co. or in such other
name as is requested by an authorized representative of the Depository (and any
payment is made to Cede & Co. or to such other entity as is requested by an
authorized representative of the Depository), ANY TRANSFER, PLEDGE, OR OTHER USE
HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL inasmuch as the
registered owner hereof, Cede & Co., has an interest herein.

                                 CITIGROUP INC.
                    FLOATING RATE NOTES DUE DECEMBER 26, 2008

REGISTERED                                                            REGISTERED

                                                              CUSIP: 172967 DF 5
                                                              ISIN: US172967DF57
                                                          Common Code: 023947684

No. R-                                                              $500,000,000

          CITIGROUP INC., a Delaware corporation (the "Company", which term
includes any successor Person under the Indenture), for value received, hereby
promises to pay to Cede & Co., or registered assigns, the principal sum of
$500,000,000 on December 26, 2008 and to pay interest thereon from and including
December 27, 2005 or from the most recent Interest Payment Date to which
interest has been paid or duly provided for, quarterly, on March 27, June 27,
September 27 and December 27 of each year, commencing March 27, 2006, at the
rate per annum for each Interest Period of three-month LIBOR, determined as
provided herein, plus 0.04% until the principal hereof is paid or made available
for payment. The interest so payable, and punctually paid or duly provided for,
on any Interest Payment Date will, as provided in the Indenture, be paid to the
Person in whose name this Note is registered at the close of business on the
Record Date for such interest, which shall be the Business Day immediately
preceding such Interest Payment Date.

<PAGE>

          Any such interest not so punctually paid or duly provided for will
forthwith cease to be payable to the holder on such Record Date and may either
be paid to the Person in whose name this Note is registered at the close of
business on a subsequent Record Date, such subsequent Record Date to be not less
than five days prior to the date of payment of such defaulted interest, notice
whereof shall be given to holders of Notes of this series not less than 15 days
prior to such subsequent Record Date, or be paid at any time in any other lawful
manner not inconsistent with the requirements of any securities exchange on
which the Notes of this series may be listed, and upon such notice as may be
required by such exchange, all as more fully provided in the Indenture.

          Interest hereon will be calculated on the basis of the actual number
of days elapsed in an Interest Period and a 360-day year. Dollar amounts
resulting from such calculation will be rounded to the nearest cent, with
one-half cent being rounded upward. An "Interest Period" shall be the period
from and including an Interest Payment Date (or from December 27, 2005 in the
case of the first Interest Payment Date) to and including the day immediately
preceding the next Interest Payment Date.

          If an Interest Payment Date falls on a day that is not a Business Day,
such Interest Payment Date will be the next succeeding Business Day. If the
Maturity of the Notes falls on a day that is not a Business Day, the payment due
on Maturity will be postponed to the next succeeding Business Day, and no
further interest will accrue in respect of such postponement. If a date for
payment of interest or principal on the Notes falls on a day that is not a
business day in the place of payment, such payment will be made on the next
succeeding business day in such place of payment as if made on the date the
payment was due. No interest will accrue on any amounts payable for the period
from and after the due date for payment of such principal or interest.

          For these purposes, "Business Day" means any day which is a day on
which commercial banks settle payments and are open for general business in The
City of New York and London.

          Payment of the principal of and interest on this Note will be made at
the office or agency of the Trustee maintained for that purpose in The City of
New York.

          Reference is hereby made to the further provisions of this Note set
forth on the reverse hereof, which further provisions shall for all purposes
have the same effect as if set forth at this place.

          Unless the certificate of authentication hereon has been executed by
the Trustee or by an authenticating agent on behalf of the Trustee by manual
signature, this Note shall not be entitled to any benefit under the Indenture or
be valid or obligatory for any purpose.


                                       2

<PAGE>

          IN WITNESS WHEREOF, the Company has caused this instrument to be duly
executed under its corporate seal.

Dated: December 27, 2005

                                        CITIGROUP INC.


                                        By:
                                            ------------------------------------
                                        Title: Chief Financial Officer

ATTEST:


By:
    ------------------------------------
Title: Assistant Secretary


                                       3

<PAGE>

          This is one of the Notes of the series issued under the
within-mentioned Indenture.

Dated: December 27, 2005

                                        THE BANK OF NEW YORK,
                                        as Trustee


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

                                        -or-

                                        CITIBANK, N.A.,
                                        as Authenticating Agent


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


                                       4

<PAGE>

     This Note is one of a duly authorized issue of Securities of the Company
(the "Notes"), issued and to be issued in one or more series under the
Indenture, dated as of March 15, 1987 (as amended and supplemented to date, the
"Indenture"), between the Company and The Bank of New York, as Trustee (the
"Trustee", which term includes any successor trustee under the Indenture), to
which Indenture and all indentures supplemental thereto reference is hereby made
for a statement of the respective rights, limitations of rights, duties and
immunities thereunder of the Company, the Trustee and the holders of the Notes
and of the terms upon which the Notes are, and are to be, authenticated and
delivered. This Note is one of the series designated on the face hereof,
initially limited in aggregate principal to $2,000,000,000.

     This Note will bear interest for each Interest Period at a rate determined
by Citibank, N.A., acting as Calculation Agent. The interest rate on this Note
for a particular Interest Period will be a per annum rate equal to three-month
LIBOR as determined on the related Interest Determination Date, plus 0.04%. The
Interest Determination Date for an Interest Period will be the second London
business day preceding such Interest Period. The Interest Determination Date for
the first Interest Period was December 22, 2005. Promptly upon determination,
the Calculation Agent will inform the Trustee and the Company of the interest
rate for the next Interest Period. Absent manifest error, the determination of
the interest rate by the Calculation Agent shall be binding and conclusive on
the holders of Notes, the Trustee and the Company.

     A London business day is a day on which dealings in deposits in U.S.
dollars are transacted in the London interbank market.

     On any Interest Determination Date, LIBOR will be equal to the offered rate
for deposits in U.S. dollars having an index maturity of six months for the next
Interest Period, in amounts of at least $1,000,000, as such rate appears on
Telerate Page 3750 at approximately 11:00 a.m., London time, on such Interest
Determination Date. If the Telerate Page 3750 is replaced by another service or
ceases to exist, the Calculation Agent will use the replacing service or such
other service that may be nominated by the British Bankers' Association for the
purpose of displaying London interbank offered rates for U.S. dollar deposits.

     If no offered rate appears on Telerate Page 3750 on an Interest
Determination Date at approximately 11:00 a.m., London time, then the
Calculation Agent (after consultation with the Company) will select four major
banks in the London interbank market and shall request each of their principal
London offices to provide a quotation of the rate at which six-month deposits in
U.S. dollars in amounts of at least $1,000,000 are offered by it to prime banks
in the London interbank market, on that date and at that time, that is
representative of single transactions at that time. If at least two quotations
are provided, LIBOR will be the arithmetic average of the quotations provided.
Otherwise, the Calculation Agent will select three major banks in New York City
and shall request each of them to provide a quotation of the rate offered by
them at approximately 11:00 a.m., New York City time, on the Interest
Determination Date for loans in U.S. dollars to leading European banks having an
index maturity of six months for the applicable Interest Period in an amount of
at least $1,000,000 that is representative of single transactions at that time.
If three quotations are provided, LIBOR will be the arithmetic average of the


                                       5

<PAGE>

quotations provided. Otherwise, the rate of LIBOR for the next Interest Period
will be set equal to the rate of LIBOR for the current Interest Period.

     The Luxembourg Stock Exchange shall be notified of the interest rate, the
amount of the interest payment and the Interest Payment Date for a particular
Interest Period not later than the first day of such Interest Period. Upon
request from any Noteholder, the Calculation Agent will provide the interest
rate in effect on this Note for the current Interest Period and, if it has been
determined, the interest rate to be in effect for the next Interest Period.

     If an event of default (as defined in the Indenture) with respect to Notes
of this series shall occur and be continuing, the principal of the Notes of this
series may be declared due and payable in the manner and with the effect
provided in the Indenture.

     The Indenture contains provisions for defeasance at any time of the entire
indebtedness of this Note upon compliance by the Company with certain conditions
set forth in Sections 11.03 and 11.04 thereof, which provisions apply to this
Note.

     The Indenture contains provisions permitting the Company and the Trustee,
without the consent of the holders of the Securities, to establish, among other
things, the form and terms of any series of Securities issuable thereunder by
one or more supplemental indentures, and, with the consent of the holders of not
less than 66 2/3% in aggregate principal amount of Securities at the time
outstanding which are affected thereby, to modify the Indenture or any
supplemental indenture or the rights of the holders of Securities of such series
to be affected, provided that no such modification will (i) extend the fixed
maturity of any Securities, reduce the rate or extend the time of payment of
interest thereon, reduce the principal amount thereof or the premium, if any,
thereon, reduce the amount of the principal of Original Issue Discount
Securities payable on any date, change the currency in which Securities are
payable, or impair the right to institute suit for the enforcement of any such
payment on or after the maturity thereof, without the consent of the holder of
each Security so affected, or (ii) reduce the aforesaid percentage of Securities
of any series the consent of the holders of which is required for any such
modification without the consent of the holders of all Securities of such series
then outstanding, or (iii) modify, without the written consent of the Trustee,
the rights, duties or immunities of the Trustee.

     No reference herein to the Indenture and no provision of this Note or of
the Indenture shall alter or impair the obligation of the Company, which is
absolute and unconditional, to pay the principal of and interest on this Note at
the times, place and rate, and in the coin or currency, herein prescribed.

     This Note is a Global Security registered in the name of a nominee of the
Depository. This Note is exchangeable for Notes registered in the name of a
person other than the Depository or its nominee only in the limited
circumstances hereinafter described. Unless and until it is exchanged in whole
or in part for definitive Notes in certificated form, this Note may not be
transferred except as a whole by the Depository to a nominee of the Depository
or by a nominee of the Depository to the Depository or another nominee of the
Depository.


                                       6

<PAGE>

     The Notes represented by this Global Security are exchangeable for
definitive Notes in certificated form of like tenor as such Notes in
denominations of $100,000 and whole multiples of $1,000 in excess thereof only
if (i) the Depository notifies the Company that it is unwilling or unable to
continue as Depository for the Notes or (ii) the Depository ceases to be a
clearing agency registered under the Securities Exchange Act of 1934, as
amended, or (iii) the Company in its sole discretion decides to allow the Notes
to be exchanged for definitive Notes in registered form. Any Notes that are
exchangeable pursuant to the preceding sentence are exchangeable for
certificated Notes issuable in authorized denominations and registered in such
names as the Depository shall direct. As provided in the Indenture and subject
to certain limitations therein set forth, the transfer of definitive Notes in
certificated form is registrable in the register maintained by the Company in
The City of New York for such purpose, upon surrender of the definitive Note for
registration of transfer at the office or agency of the registrar, duly endorsed
by, or accompanied by a written instrument of transfer in form satisfactory to
the Company and the registrar duly executed by, the holder thereof or his
attorney duly authorized in writing, and thereupon one or more new Notes of this
series and of like tenor, of authorized denominations and for the same aggregate
principal amount, will be issued to the designated transferee or transferees.
Subject to the foregoing, this Note is not exchangeable, except for a Global
Security or Global Securities of this issue of the same principal amount to be
registered in the name of the Depository or its nominee.

     No service charge shall be made for any such registration of transfer or
exchange, but the Company may require payment of a sum sufficient to cover any
tax or other governmental charge payable in connection therewith.

     Prior to due presentment of this Note for registration of transfer, the
Company, the Trustee and any agent of the Company or the Trustee may treat the
Person in whose name this Note is registered as the owner hereof for all
purposes, whether or not this Note be overdue, and neither the Company, the
Trustee nor any such agent shall be affected by notice to the contrary.

     The Company will pay additional amounts ("Additional Amounts") to the
beneficial owner of any Note that is a non-United States person in order to
ensure that every net payment on such Note will not be less, due to payment of
U.S. withholding tax, than the amount then due and payable. For this purpose, a
"net payment" on a Note means a payment by the Company or a paying agent,
including payment of principal and interest, after deduction for any present or
future tax, assessment or other governmental charge of the United States. These
Additional Amounts will constitute additional interest on the Note.

     The Company will not be required to pay Additional Amounts, however, in any
of the circumstances described in items (1) through (13) below.

     (1)  Additional Amounts will not be payable if a payment on a Note is
          reduced as a result of any tax, assessment or other governmental
          charge that is imposed or withheld solely by reason of the beneficial
          owner:


                                       7

<PAGE>

          (a)  having a relationship with the United States as a citizen,
               resident or otherwise;

          (b)  having had such a relationship in the past or

          (c)  being considered as having had such a relationship.

     (2)  Additional Amounts will not be payable if a payment on a Note is
          reduced as a result of any tax, assessment or other governmental
          charge that is imposed or withheld solely by reason of the beneficial
          owner:

          (a)  being treated as present in or engaged in a trade or business in
               the United States;

          (b)  being treated as having been present in or engaged in a trade or
               business in the United States in the past or

          (c)  having or having had a permanent establishment in the United
               States.

     (3)  Additional Amounts will not be payable if a payment on a Note is
          reduced as a result of any tax, assessment or other governmental
          charge that is imposed or withheld in whole or in part by reason of
          the beneficial owner being or having been any of the following (as
          such terms are defined in the Internal Revenue Code of 1986, as
          amended):

          (a)  personal holding company;

          (b)  foreign personal holding company;

          (c)  foreign private foundation or other foreign tax-exempt
               organization;

          (d)  passive foreign investment company;

          (e)  controlled foreign corporation or

          (f)  corporation which has accumulated earnings to avoid United States
               federal income tax.

     (4)  Additional Amounts will not be payable if a payment on a Note is
          reduced as a result of any tax, assessment or other governmental
          charge that is imposed or withheld solely by reason of the beneficial
          owner owning or having owned, actually or constructively, 10 percent
          or more of the total combined voting power of all classes of stock of
          the Company entitled to vote or by reason of the beneficial owner
          being a bank that has invested in a Note as an extension of credit in
          the ordinary course of its trade or business.

For purposes of items (1) through (4) above, "beneficial owner" means a
fiduciary, settlor, beneficiary, member or shareholder of the holder if the
holder is an estate, trust, partnership, limited liability company, corporation
or other entity, or a person holding a power over an estate or trust
administered by a fiduciary holder.

     (5)  Additional Amounts will not be payable to any beneficial owner of a
          Note that is a:

          (a)  fiduciary;


                                       8

<PAGE>

          (b)  partnership;

          (c)  limited liability company or

          (d)  other fiscally transparent entity

          or that is not the sole beneficial owner of the Note, or any portion
          of the Note. However, this exception to the obligation to pay
          Additional Amounts will only apply to the extent that a beneficiary or
          settlor in relation to the fiduciary, or a beneficial owner or member
          of the partnership, limited liability company or other fiscally
          transparent entity, would not have been entitled to the payment of an
          Additional Amount had the beneficiary, settlor, beneficial owner or
          member received directly its beneficial or distributive share of the
          payment.

     (6)  Additional Amounts will not be payable if a payment on a Note is
          reduced as a result of any tax, assessment or other governmental
          charge that is imposed or withheld solely by reason of the failure of
          the beneficial owner or any other person to comply with applicable
          certification, identification, documentation or other information
          reporting requirements. This exception to the obligation to pay
          Additional Amounts will only apply if compliance with such reporting
          requirements is required by statute or regulation of the United States
          or by an applicable income tax treaty to which the United States is a
          party as a precondition to exemption from such tax, assessment or
          other governmental charge.

     (7)  Additional Amounts will not be payable if a payment on a Note is
          reduced as a result of any tax, assessment or other governmental
          charge that is collected or imposed by any method other than by
          withholding from a payment on a Note by the Company or a paying agent.

     (8)  Additional Amounts will not be payable if a payment on a Note is
          reduced as a result of any tax, assessment or other governmental
          charge that is imposed or withheld by reason of a change in law,
          regulation, or administrative or judicial interpretation that becomes
          effective more than 15 days after the payment becomes due or is duly
          provided for, whichever occurs later.

     (9)  Additional Amounts will not be payable if a payment on a Note is
          reduced as a result of any tax, assessment or other governmental
          charge that is imposed or withheld by reason of the presentation by
          the beneficial owner of a Note for payment more than 30 days after the
          date on which such payment becomes due or is duly provided for,
          whichever occurs later.

     (10) Additional Amounts will not be payable if a payment on a Note is
          reduced as a result of any:

          (a)  estate tax;

          (b)  inheritance tax;

          (c)  gift tax;


                                       9

<PAGE>

          (d)  sales tax;

          (e)  excise tax;

          (f)  transfer tax;

          (g)  wealth tax;

          (h)  personal property tax or

          (i)  any similar tax, assessment, withholding, deduction or other
               governmental charge.

     (11) Additional Amounts will not be payable if a payment on a Note is
          reduced as a result of any tax, assessment, or other governmental
          charge required to be withheld by any paying agent from a payment of
          principal or interest on a Note if such payment can be made without
          such withholding by any other paying agent.

     (12) Additional amounts will not be payable if a payment on a Note is
          reduced as a result of any tax, assessment or other governmental
          charge that is required to be made pursuant to any European Union
          directive on the taxation of savings income or any law implementing or
          complying with, or introduced to conform to, any such directive.

     (13) Additional Amounts will not be payable if a payment on a Note is
          reduced as a result of any combination of items (1) through (12)
          above.

     Except as specifically provided herein, the Company will not be required to
make any payment of any tax, assessment or other governmental charge imposed by
any government or a political subdivision or taxing authority of such
government.

     As used in this Note, "United States person" means:

     (a)  any individual who is a citizen or resident of the United States;

     (b)  any corporation, partnership or other entity created or organized in
          or under the laws of the United States;

     (c)  any estate if the income of such estate falls within the federal
          income tax jurisdiction of the United States regardless of the source
          of such income and

     (d)  any trust if a United States court is able to exercise primary
          supervision over its administration and one or more United States
          persons have the authority to control all of the substantial decisions
          of the trust.

     Additionally, "non-United States person" means a person who is not a United
States person, and "United States" means the states of the United States of
America and the District of Columbia, but excluding its territories and its
possessions.

     Except as provided below, the Notes may not be redeemed prior to maturity.

     (1)  The Company may, at its option, redeem the Notes if:


                                       10

<PAGE>

          (a)  the Company becomes or will become obligated to pay Additional
               Amounts as described above;

          (b)  the obligation to pay Additional Amounts arises as a result of
               any change in the laws, regulations or rulings of the United
               States, or an official position regarding the application or
               interpretation of such laws, regulations or rulings, which change
               is announced or becomes effective on or after December 19, 2005
               and

          (c)  the Company determines, in its business judgment, that the
               obligation to pay such Additional Amounts cannot be avoided by
               the use of reasonable measures available to it, other than
               substituting the obligor under the Notes or taking any action
               that would entail a material cost to the Company.

     (2)  The Company may also redeem the Notes, at its option, if:

          (a)  any act is taken by a taxing authority of the United States on or
               after December 19, 2005, whether or not such act is taken in
               relation to the Company or any affiliate, that results in a
               substantial probability that the Company will or may be required
               to pay Additional Amounts as described above;

          (b)  the Company determines, in its business judgment, that the
               obligation to pay such Additional Amounts cannot be avoided by
               the use of reasonable measures available to it, other than
               substituting the obligor under the Notes or taking any action
               that would entail a material cost to the Company and

          (c)  the Company receives an opinion of independent counsel to the
               effect that an act taken by a taxing authority of the United
               States results in a substantial probability that the Company will
               or may be required to pay the Additional Amounts described above,
               and delivers to the Trustee a certificate, signed by a duly
               authorized officer, stating that based on such opinion the
               Company is entitled to redeem the Notes pursuant to their terms.

Any redemption of the Notes as set forth in clauses (1) or (2) above shall be in
whole, and not in part, and will be made at a redemption price equal to 100% of
the principal amount of the Notes Outstanding plus accrued interest thereon to
the date of redemption. Holders shall be given not less than 30 days nor more
than 60 days prior notice by the Trustee of the date fixed for such redemption.

     All terms used in this Note which are defined in the Indenture shall have
the meanings assigned to them in the Indenture. The Notes are governed by the
laws of the State of New York.


                                       11