The spermatophore morphology and spermatozoal ultrastructure of Diogenes pugilator are described. The spermatophore is typically tripartite made up of an ampulla, a very long stalk and a foot. The two halves making up the ampulla meet at a lateral ridge. The sperm is ovoidal in shape and composed of a large, almost cylindrical, acrosome, which occupies most of the cell volume, capped by a conical operculum, and cupped posteriorly by a cytoplasmic region comprising the nucleus and three arms. The sizes of the different parts of the spermatophore and of the sperm are given and their exterior morphology and ultrastructure described and compared to congeners. The available spermatozoal and spermatophore data is used to place D. pugilator within a sperm phylogeny of the hermit crab family Diogenidae.