Menidia clarkhubbsi, an all-female atherinid fish, was recently discovered on the Gulf Coast of Texas where it coexists with two similar, bisexual (=gonochoristic herein) species, M. beryllina and M. peninsulae (Echelle and Mosier, 1981, 1982). Excepting Asian and Japanese populations of silver carp and goldfish (family Cyprinidae), which may be man-induced (Schultz, 1977), all previously known, naturally occurring populations of unisexual fishes are livebearers (family Poeciliidae) of the genera Poecilia and Poeciliopsis. Knowledge of the genetic structure and evolution of unisexual fishes is confined largely to poeciliids (e.g., Vrijenhoek et al., 1977, 1978; Turner et al., 1980b). Most well-studied unisexual vertebrates apparently originated as interspecific hybrids (Schultz, 1969; Uzzell and Berger, 1975; Uzzell and Darevsky, 1975; Parker and Selander, 1976; Turner et al., 1980a). These unisexual forms have mechanisms which prevent genetic recombination, cause clonal population structure, and tend to preserve hybrid genomes. In this paper we present an allozymic survey of M. clarkhubbsi and its bisexual relatives. Our purposes were to determine whether the genetic structure of the unisexual species is explained by interspecific hybridization, and, if so, to ask which bisexual forms are the most likely parental species. Our data, in combination with previous, geographically extensive, allozymic surveys of Menidia (Johnson, 1975; Duggins, 1980), suggest that M. clarkhubbsi arose as a hybrid between M. beryllina-like and M.