During spermiogenesis in the sea urchinsDiadema setosumandSalmacis bicolor, two pattems of flagellum development were shown. For both species a significant proportion of spermatids examined possessed an axoneme that remained associated with the nucleus during development, and this results in the formation of sperm cells with an anteriorly directed flagellum, whereas the majority of spermatids gave rise to sperm cells with a posterior flagellum which is typical of multicellular animals. The morphology ofD. setosumandS. bicolorsperm cells was generally similar to that of other echinoids.