29 C.F.R. § 825.104 : Labor — REGULATIONS RELATING TO LABOR — WAGE AND HOUR DIVISION, DEPARTMENT OF LABOR — OTHER LAWS — THE FAMILY AND MEDICAL LEAVE ACT OF 1993 — COVERAGE UNDER THE FAMILY AND MEDICAL LEAVE ACT — Covered employer

(a) An employer covered by FMLA is any person engaged in commerce or in any industry or activity affecting commerce, who employs 50 or more employees for each working day during each of 20 or more calendar workweeks in the current or preceding calendar year. Employers covered by FMLA also include any person acting, directly or indirectly, in the interest of a covered employer to any of the employees of the employer, any successor in interest of a covered employer, and any public agency. Public agencies are covered employers without regard to the number of employees employed. Public as well as private elementary and secondary schools are also covered employers without regard to the number of employees employed. See§825.600.
(b) The terms commerce and industry affecting commerce are defined in accordance with section 501(1) and (3) of the Labor Management Relations Act of 1947 (LMRA) ( 29 U.S.C. 142(1) and (3) ), as set forth in the definitions at §825.102 of this part. For purposes of the FMLA, employers who meet the 50-employee coverage test are deemed to be engaged in commerce or in an industry or activity affecting commerce.
(c) Normally the legal entity which employs the employee is the employer under FMLA. Applying this principle, a corporation is a single employer rather than its separate establishments or divisions.
(1) Where one corporation has an ownership interest in another corporation, it is a separate employer unless it meets the joint employment test discussed in §825.106, or the integrated employer test contained in paragraph (c)(2) of this section.
(2) Separate entities will be deemed to be parts of a single employer for purposes of FMLA if they meet the integrated employer test. Where this test is met, the employees of all entities making up the integrated employer will be counted in determining employer coverage and employee eligibility. A determination of whether or not separate entities are an integrated employer is not determined by the application of any single criterion, but rather the entire relationship is to be reviewed in its totality. Factors considered in determining whether two or more entities are an integrated employer include:
(i) Common management;
(ii) Interrelation between operations;
(iii) Centralized control of labor relations; and
(iv) Degree of common ownership/financial control.
(d) An employer includes any person who acts directly or indirectly in the interest of an employer to any of the employer's employees. The definition of employer in section 3(d) of the Fair Labor Standards Act (FLSA), 29 U.S.C. 203(d), similarly includes any person acting directly or indirectly in the interest of an employer in relation to an employee. As under the FLSA, individuals such as corporate officers "acting in the interest of an employer" are individually liable for any violations of the requirements of FMLA.

29 C.F.R. § 825.104

78 FR 8902, 2/6/2013, as amended at 82 FR 2230, 1/9/2017