The ultrastructure of spermatogenesis and spermatozoon of Coelomactra antiquata was observed with transmission electron microscope. Detailed observations and descriptions of morphological and structural changes of cells as well as transformat ion of cell organells were made submicroscopically from spermatogonia to spermatozoa. The spermat id differentiat ion can be divided into six stages. During spermatid differentiation, the main changes are as follow: elongation of nucleus, condensation of chromatin( in granular pattern) , acrosome format ion, development and fusion of mitochondria centriole shifting, and flagellar formation cytoplasmic sloughing. Mature sperms are of typical primitive type, consisting of three regions: head, middle piece and tail. The acrosome is conical. Acrosomal materials w ith a lampshade-shaped structure lie round its base. The subacrosome space is conical w ith materials of low density. The nucleus is ellipsoidal. Four large ellipsoidal mitochondria and two centrioles make up the middle piece. The flagellum is a standard / 9+ 20 microtubular structure.