The structure of the spermatozoa of four species ofMelanopsis(Melanopsidae) is described by light and transmission electron microscopy. All species produce two types of spermatozoa, euspermatozoa (fertilizing) and paraspermatozoa (non-fertilizing). The eusperm of all species are about 42–44 μm long and have a morphology similar to that of other cerithioideans. The head is composed of a laterally flattened nucleus and conical acrosome of approximately equal size. The nucleus has a shallow posterior fossa which houses a centriolar derivative from which the axoneme emerges. The four equal-sized mitochondria of the midpiece are non-helically arranged around the axoneme. Each mitochondrion has an anterior flange and the cristae are plate-like. This midpiece arrangement suggests that the Melanopsidae are more closely related to the Modulidae, Potamididae and possibly Pachychilidae than to other cerithiacean taxa. Posterior to the midpiece is a 10–12 μm-long glycogen piece in which the glycogen granules encircle the axoneme as inner and outer rings. Although the structure of the euspermatozoa is very similar in all four species, there are significant differences in the lengths of the nucleus, acrosome and midpiece between species. These dimensional differences suggest that sperm morphology could be of value in the separation and identification of species within the genusMelanopsis.The oligopyrenic parasperm are about 65 μm long and consist of a head and tail region of multiple flagella. The axonemes of the flagella are positioned centrally within the head and run for its length. Spermiogenesis (both eu- and paraspermiogenesis) is as described for other caenogastropods. During sperm development, spermatids are closely associated with Sertoli cells. Unlike previous reports for some caenogastropods, the Sertoli cells do not completely surround the early spermatids. Instead, long tubular cytoplasmic processes from the Sertoli cells encircle the developing spermatozoa. These processes contain microfilaments which are thought to be composed of actin.