Chorionic gonadotropin (CG) is a critical signal in establishing pregnancy in humans and some other primates, but this placentally expressed hormone has not been found in other mammalian orders. The gene for one of its two subunits (CG beta subunit [CGbeta]) arose by duplication from the luteinizing hormone beta subunit gene (LHbeta), present in all mammals tested. In this study, 14 primate and related mammalian species were examined by Southern blotting and DNA sequencing to determine where in mammalian phylogeny the CGbeta gene originated. Bats (order Chiroptera), flying lemur (order Dermoptera), strepsirrhine primates, and tarsiers do not have a CGbeta gene, although they possess one copy of the LHbeta gene. The CGbeta gene first arose in the common ancestor of the anthropoid primates (New World monkeys, Old World monkeys, apes, and humans), after the anthropoids diverged from tarsiers. At least two subsequent duplication events occurred in the catarrhine primates, all of which possess multiple CGbeta copies. The LHbeta-CGbeta family of genes has undergone frequent gene conversion among the catarrhines, as well as periods of strong positive selection in the New World monkeys (platyrrhines). In addition, newly generated DNA sequences from the promoter of the CG alpha subunit gene indicate that platyrrhine monkeys use a different mechanism of alpha gene expression control than that found in catarrhines.