Unisexual poeciliid fishes live as sexual parasites in breeding complexes with related bisexual species. Males of the host species copulate with unisexual females as well as with conspecifics, thus maintaining the unisexuals. Copulation with a unisexual offers no selective benefit for a male. A model is proposed that provides an explanation in terms of evolutionary ecology for why males copulate with unisexuals. It assumes that, before copulation, a male attempt to identify a female as conspecific or not but that the correctness of the identification depends on the length of time spent on identification process. Some cost is involved in the passage of time, so an optimal time spent on identification must exist. Because subordinate males risk being driven away by dominant males, the optimal time is longer for males at the top of the dominance hierarchy than for males at the bottom. Such an optimal strategy gives a male the greatest possible average net benefit from a mating attempt, given his social status ; this is a « best of a bad job » strategy.