The cyprid larvae ofScalpellum scalpellum (L.), a hermaphroditic goose barnacle with dwarf males, are morphologically alike and no characters distinguish the sexes. To elucidate the controversy over the relative significance of genetic and environmental factors in the sex determination of this species, laboratory experiments were performed involving the introduction of cyprid larvae of different parental origin to different densities of adult hermaphrodites without dwarf males. Cyprids introduced to adult hermaphrodites metamorphosed to either dwarf males or hermaphrodites significantly different from the controls without adult hermaphrodites. The proportion which metamorphosed into dwarf males increased with increasing adult density. Not more than 51% metamorphosis to dwarf males could be obtained, indicating a genetic component in the sex differentiation. It was concluded that all cyprids are potential hermaphrodites, but that about 50% possess the capability of metamorphosing into dwarf males as well, if adult hermaphrodites with free receptacles are available. Consequently, both genetically and environmentally controlled components influence sex determination in this species.