We, the National Marine Fisheries Service (NMFS), issue final regulations to amend the Code of Federal Regulations (CFR) to implement the Endangered Species Act (ESA) regarding experimental populations. This rule amends the CFR to establish definitions and procedures for: Establishing and/or designating certain populations of species otherwise listed as endangered or threatened as experimental populations; determining whether experimental populations are "essential" or "nonessential;" and promulgating appropriate protective measures for experimental populations.