Per the Federal Register:
The American alligator first received protection under Federal law in 1967 when it was listed as endangered throughout its range under the Endangered Species Preservation Act of 1966 (32 FR 4001, March 11, 1967), a predecessor to the Endangered Species Act of 1973, as amended (16 U.S.C. 1531 et seq. (Act, ESA)). Its endangered classification was transferred to the Act effective December 28, 1973, (Pub. L. 93-205, 1, Dec. 28, 1973, 87 Stat. 884).
Thanks.