After males of Lasioglossum zephyrum (Hymenoptera: Halictidae) have first been exposed to a conspecific female, the attractiveness of a second female diminishes with increasing genealogical relationship between the two females. The most parsimonious explanation for this is a polygenically controlled female sex pheromone. Nonrandom mating preferences may markedly affect gene flow within and between natural populations. Polygenically controlled sex pheromones probably also enhance kin recognition among females and, thus, influence the evolution of eusocial behavior in bees and wasps.