Histological observations on gonadal formation and hermaphroditism in 16 species (4 genera) of Japanese butterflyfishes (Chaetodontidae), included a detailed study of the former in Chaetodon auripes from central Japan. In this species, the gonads of 24 young specimens (<36.4 mm SL) were sexually undeter mined, whereas those of 16 larger specimens (39.4 mm SL to 55.5 mm SL) had begun initial differentiation into an ovary, having formed an early parovarian type ovarian cavity. Sex changes occurred in immature ovaries, which initially became intersexual gonads with many seminal lobules forming in the ovigerous lamellae and eventually matured into secondary testes (each concurrently retaining an ovar ian cavity). No specimens were found with gonads that had developed directly into a testis from a sexually undifferentiated stage. In Forcipiger flavissimus, three stages of gonadal sex succession, viz., ovaries with a parovarian type ovarian cav ity, intersexual gonads and secondary testes with a rudimentary ovarian cavity, were found. In Hemitaurichthys polylepis and Heniochus diphreutes, intersexual gonads of both ovarian-and testicular-types were found. Accordingly, the occur rence of normal, although non-functional, hermaphroditism and sex change from female to male is now established in all of the Japanese chaetodontid species stud ied.
