Spermatogenesis and sperm ultrastructure of the marsh clam (Corbicula japonica) were investigated by electron microscopic observations. Testis of the marsh clam consists of numerous spermatogenic follicle containing germ cells in the different developmental stages. Spermatogonia are located nearest the outer wall of the follicle, while spermtocytes and spermatids are positioned nearer to the lumen. Spematogonia are oval-shaped and about $5{mu}m$ In diameter. Spermatogonia develop into spermatocyte, spermatid and spermatozoon. In the spermatid to about $2{mu}m$ in diameter, cytoplasm decreases and mitochondria move to the base of the nucleus and fuse into several spheres, the centrioles become orthogonally oriented, a flagellum appears, and an acrosomal vesicle forms. The mature sperm has primitive type, consisting of a head, a midpiece and a tail. The sperm was arrow-shaped, and its head is about $8{mu}m$ long and comprised of a long nucleus and an acrosome. The four mitochondria encircled the centrosome in midpiece. The flagellum had the classical 9+2 axoneme structure, and axonemal lateral fins in the tail were observed.