The ultrastructure of the spermatozoon was examined with electron microscopy in Patinopecten yessoensis.The spermatozoon is typically primitive,with its body length and head length about 50 μm and 3 μm,respectively.The mature spermatozoon consists of head,mid-piece and tail.The acrosome is in upside-down V-shape.The nucleus,which consists of major part of the head,is egg-shaped with high density.It contains anterior nuclear fossa,posterior nuclear fossa and implantation fossa;the centriolar complex which is surrounded by 4-5 oval-shaped mitochondria,forms the mid-piece.The flagellum is a typical "9+2" microtubular structure.