Population structure and gonad histology were investigated in six species of the anemonefish genus Amphiprion occurring in Japanese waters.Species investigated included Amphiprion frenatus, A.clarkii, A.polymnus, A.perideraion, A.sandracinos, and A.ocellaris.All six species occur around their host anemones in small social groups that include a monogamous pair of sexually functional adults and from zero to three sexually non-functional subadults and juveniles.In all species, the single female in a particular social group is larger than her mate, considerably so in some species.Histological examination of gonads indicated that all individuals of all six species are ambosexual as juveniles.At first only testicular tissues mature.Under certain conditions these may later degenerate and the gonads change to ovaries.Field studies supported histological results.A strict dominance hierarchy exists in each social group.The largest fish is always the single female, and the second largest fish is invariably a functional male.Only the two largest fish are sexually functional.Two functional males of A.clarkii under study in the field changed to females after the loss of their previous mates.The six Amphiprion species in Japan present examples of socially controlled protandrous sex inversion.