The ultrastructural characteristics of germ cell differentiations during spermatogenesis and mature sperm morphology in male $Atrina$ ($Servatrina$) $pectinata$ were evaluated via transmission electron microscopic observation. The accessory cells, which contained a large quantity of glycogen particles and lipid droplets in the cytoplasm, are assumed to be involved in nutrient supply for germ cell development. Morphologically, the sperm nucleus and acrosome of this species are ovoid and conical in shape, respectively. The acrosomal vesicle, which is formed by two kinds of electron-dense or lucent materials, appears from the base to the tip: a thick and slender elliptical line, which is composed of electron-dense opaque material, appears along the outer part (region) of the acrosomal vesicle from the base to the tip, whereas the inner part (region) of the acrosomal vesicle is composed of electron-lucent material in the acrosomal vesicle. Two special characteristics, which are found in the acrosomal vesicle of A. ($S$) $pectinata$ in Pinnidae (subclass Pteriomorphia), can be employed for phylogenetic and taxonomic analyses as a taxonomic key or a significant tool. The spermatozoa were approximately $45-50{mu}m$ in length, including a sperm nucleus (about $1.43{mu}m$ in length), an acrosome (about $0.51{mu}m$ in length), and a tail flagellum (about $46-47{mu}m$). The axoneme of the sperm tail evidences a 9+2 structure.