Abstract: The histology and ultrastructure of male reproductive system of mussels Mytilus edulis were studied with light microscope and transmission electron microscope.The male reproductive system is composed of branch testis and vas deferens.The testis consist of great number of branchy seminiferous tubules and capsules.The tubule showed by the transverse section is made up of germinative epithelia (spermatogonium) and tubule liner,and the germinative epithelia constantly produce spermatocyte,spermatid and sperm.The spermatogonium has an oval shape,in which there are dispersed chromatin,mitochondria and many free ribosomes.In primary spermatocyte and secondary spermatocyte the chromatin densified to some blocks.In the early spermatid,many small dense particulates which produced from rough endoplasmic reticulum vesicles,aggregated to form a large proacrosomal vesicles.The acrosomal vesicle futher develop into "Λ"shape acrosome which composed of apical cap,acrosomal central tubule and lamellar structure.At the same time,the nucleus and flagellum were produced (there are 5 mitochondria in base of the flagellum) and the sperm matured.