26 CFR § 31.6011(b)-1 - Employers' identification numbers.
(a) Requirement of application—(1) In general—(i) Before October 1, 1962. Except as provided in paragraph (b) of this section, every employer who on any day after December 31, 1954, and before October 1, 1962, has in his employ one or more individuals in employment for wages subject to the taxes imposed by the Federal Insurance Contributions Act, but who prior to such day neither has been assigned an identification number nor has applied therefor, shall make an application on Form SS–4 for an identification number.
(ii) On or after October 1, 1962. Except as provided in paragraph (b) of this section, every employer who on any day after September 30, 1962, has in his employ one or more individuals in employment for wages which are subject to the taxes imposed by the Federal Insurance Contributions act or which are subject to the withholding of income tax from wages under section 3402, but who prior to such day neither has been assigned an identification number nor has applied therefor, shall make an application on Form SS–4 for an identification number.
(iii) Method of application. The application, together with any supplementary statement, shall be prepared in accordance with the form, instructions, and regulations applicable thereto, and shall set forth fully and clearly the data therein called for. Form SS–4 may be obtained from any district director or director of a service center or any district office of the Social Security Administration. The application shall be filed with the internal revenue officer designated in the instructions applicable to Form SS-4, or with the nearest district office of the Social Security Administration. The application shall be signed by (a) the individual, if the employer is an individual; (b) the president, vice president, or other principal officer, if the employer is a corporation; (c) a responsible and duly authorized member or officer having knowledge of its affairs, if the employer is a partnership or other unincorporated organization; or (d) the fiduciary, if the employer is a trust or estate. An identification number will be assigned to the employer in due course upon the basis of the information reported on the application required under this section.
(2) Time for filing Form SS–4. The application for an identification number shall be filed on or before the seventh day after the first payment of wages to which reference is made in paragraph (a)(1) of this section. For provisions relating to the time when wages are paid, see § 31.3121(a)–2 and paragraph (b) of § 31.3402(a)–1.
(b) Employers who are assigned identification numbers without application. An identification number may be assigned, without application by the employer, in the case of an employer who has in his employ only employees who are engaged exclusively in the performance of domestic service in his private home not on a farm operated for profit (see § 31.3121(a)(7)–1. If an identification number is so assigned, the employer is not required to make an application on Form SS–4 for the number.
(c) Crew leaders. Any person who, as a crew leader within the meaning of section 3121(o), furnishes individuals to perform agricultural labor for another person shall, on or before the first date on which he furnishes such individuals to perform such labor for such other person, advise such other person of his name; permanent mailing address, or if none, present address; and identification number, if any.
(d) Use of identification number. The identification number assigned to an employer (other than a household employer referred to in paragraph (b) of this section) shall be shown in the employer's records, and shall be shown in his claims to the extent required by the applicable forms, regulations, and instructions. For provisions relating to the inclusion of identification numbers in returns, statements on Form W-2, and depositary receipts, see § 31.6109–1.