Abstract Spermatogenesis and Spenniogenesis in the monocotylid monogenean Calicotyle kroyeri are basically similar to those of the related monocotylid Heterocotyle sp. but there are two major differences in the ultrastructure of the spermatozoon. First, typical spermatozoa of Calicotyle have two fully developed axonemes; occasionally spermatozoa are found with three axonemes and there is evidence that this condition arises during Spenniogenesis rather than by later fusion of spermatozoa. Secondly, Calicotyle spermatozoa possess at the ‘anterior’ end, a complex, possibly spiral end-piece at the time of release of the fully developed sperm into the testis lumen. This end-piece is lost, probably before the spermatozoa reach the vas deferons. The phylogenetic implications of these findings are briefly considered.