Unisexual forms of Poeciliopsis require sperm from males of gonochoristic species. Selection pressures dictate that males discriminate against unisexuals until the latter become extinct. Most Poeciliopsis unisexual—bisexual populations have a unisexual—bisexual female ratio that favors unisexuals, indicating that selection for conspecific mating by males has either not been operative, not persisted for a sufficient period of time, or been disrupted by unknown factors. In mate selection and insemination tests single males show almost absolute preference for conspecific females, but social interactions among males result in unisexual inseminations. Mate selection reduces unisexual fecundity in natural populations, but the all—female forms receive sufficient sperm to sustain themselves.