Ultrastructural features of euspermiogenesis and euspermatozoa inCerithium caeruleumare described. Early spermatids possess round eccentric nuclei with fine chromatin material. The cytoplasm contains sparse endoplasmic reticulum, few mitochondria, and a well developed Golgi complex with which a proacrosomal vesicle is associated. Acrosome differentiation starts with a U-shaped vesicle plugged with a dense interstitial granule. Later, the vesicle becomes flask-shaped with plate-like substructures in its core and external supporting structures at its margins. The early oval nucleus becomes hammer-head and eventually bullet-shaped with a posterior invagination accommodating the proximal portion of the axoneme. Chromatin changes from fine granular to fibrillar, lamellar, and finally a complete dense material. The mature flat acrosome consists of a cone with internal plate-like substructures, a rod, and a basal plate. The middle piece comprises two large and two extremely small non-helically arranged mitochondria, which were derived from four large mitochondrial spheres in early spermatids. Microtubules surround the nucleus and the middle piece at late stages of their development. A dense ring structure separates the middle piece from the glycogen piece. The latter consists of nine tracts of glycogen granules associated with the axoneme. Based on fine features of euspermiogenesis, we suggest that Cerithiacea may represent an evolutionary link between Archaeogastropoda on one hand, and Mesogastropoda and Neogastropoda on the other. Also, we believe that euspermatozoan ultrastructure is useful in building taxonomic affinities between major groups of gastropods at and above the family level, with no consistent species-specific characters.