As in many prosobranchs, two cell lines develop in the testis ofStrombus gigas. The typical spermatogenesis is similar to that described in many species. The spermatic cell development is characterized by cell divisions. Typical spermatogenesis leads to a mature spermatozoa with a column-shaped nucleus. The acrosome consists of an acrosomal rod covered by an acrosomal cone and topped by a hemispherical vesicle. The middle piece is made up of nine mitochondria helically arranged around the axoneme. A ring centriole is present between the middle piece and the flagellum. The tail contains the axoneme and glycogen granules. During atypical spermatogenesis no division of the cells is observed. The cytoplasm of atypical spermatogonia increases in size while the nucleus degenerates. In the cytoplasm dark granules and vesicles with double membranes develop. The mature atypical spermatozoa are spindle-shaped. The cytoplasm contains large dark granules, but no remnants of the vesicles with double membranes are observed. Each side of the atypical spermatozoa is formed by an undulating membrane containing numerous axonemes. The possible functions of the atypical spermatozoa are discussed in this paper.