Abstract: The ultrastructure of spermatogenesis of Bullacta exarata was studied by using of transmission electron microscopy.The results show that there is a series of considerable changes in morphology and structure during the spermatogenesis.These changes include gradual nuclear elongation,chromatin condensation,formation of acrosome,development and fusion of mitochondria,cytoplasmic sloughing and flagellar formation.Acrosome is formed from proacrosomal vesicle fused with vesicles secreted by Golgi apparatus.Axoneme is formed from centriole.Spermatid differentiation can be divided into three stages.During the differentiation,the morphology and structure of nucleus and chromation are different from other Mollusks.The nucleus of early spermatid is oval-shaped and comprises heterogeneous aggregations of flocculent granular chromatin.In the middle stage,it is kidney shaped and its chromatin condenses to finely fibrils-shaped.As the spermatid matures,the nucleus elongates in the anterior-posterior axis,and the chromatin condenses to long fibrils and then condenses to high and even electron-density till the formation of spermatozoon nucleus.At the same time,the nucleus transforms from cylindric shape to long cylindric shape.Significance of nucleus and cell organelle as taxonomic character in the spermatogenesis of B.exarata is roughly discussed.