Ultrastructural changes during spermatogenesis of Scapharca broughtoni were investigated by transmission electron microscopy. Results showed that spermatogenesis in this species underwent successive stages of spermatogonium, primary spermatocyte, secondary spermatocyte, spermatid and spermatozoa. The changes during spermiogenesis mainly include acrosome development, morphological transformation of nucleus and formation of tail. In these processes, proacrosomal granules accumulated and fused into acrosomal vesicle and finally became a coneshaped acrosome. The nucleus changed from round or oval to tympaniform. The chromatin in nucleus changed gradually from agglomerate to granule and eventually became homogenous with high electron density. Mitochondria accumulated and fused with larger size. At the same time, they gradually moved to the nucleus posterior and participated in formation of the midpiece. Mature spermatozoon contained three parts: head, midpiece and tail. The head consisted of an acrosome and a nucleus. The midpiece comprised five mitochondria surrounding the distal centriole. The tail was a slender flagellum.