ABSTRACTSperm morphology offers a rich source of potentially informative characters for gastropod taxonomy and in this study it was used to provide additional information for Gyraulus (Armiger) crista (Linnaeus, 1758), whose taxonomic position within the Planorbidae Rafinesque, 1815 remains uncertain. Sperm morphology in G. (A.) crista and G. (Gyraulus) albus (Müller, 1774), the type species of the genus, was examined using light microscopy, scanning electron microscopy and transmission electron microscopy. The ultrastructure of spermatozoa in the two species was found to conform to the general pattern characteristic of the Hygrophila Férussac, 1822: the spermatozoa had an anterior head containing an acrosomal complex (apical vesicle and acrosomal pedestal) and cone-shaped nucleus, followed by a very long midpiece with glycogen spirals, and then by a glycogen piece and tail. Although the spermatozoa of the two species shared some common features (one nuclear keel and four glycogen spirals), they differed significantly in length, morphology of the annulus and structure of the spiral surface ridges. There were also some differences in the size and structure of the acrosomal pedestal and the shape of the nucleus. The spermatozoa of the two Gyraulus species are compared with those of other Hygrophila and the possible importance of sperm morphology for the taxonomic status of G. (A.) crista is discussed.