AbstractThe taxonomic position of the Acroloxidae Thiele, 1931 within the Lymnaeoidea Rafinesque, 1815 remains ambiguous. This study is an attempt to evaluate the possibility of using sperm morphology and ultrastructure in resolving the phylogenetic relationships of the Acroloxidae based on a microscopic examination of spermatozoa in Acroloxus lacustris (Linnaeus, 1758), a common Eurasian species, using light, scanning electron and transmission electron microscopy. Acroloxus lacustris, like other Lymnaeoidea, have spermatozoa divided into the head, midpiece, glycogen region and endpiece. The sperm head contains an extremely long nucleus carrying three rounded keels and a cylindrical acrosome composed of an apical vesicle and homogeneously electron‐dense pedestal. The subnuclear ring is an electron‐dense cylinder sandwiched between the basal projection of the nucleus and mitochondrial derivative. The midpiece contains the mitochondrial derivative enclosing apically a single glycogen helix. The spermatozoa have a well‐developed glycogen region and an unusual endpiece with an axoneme anchored on an electron‐dense plaque. Several sperm characters of A. lacustris (solid acrosomal pedestal, rounded nuclear keels, single glycogen helix) differ from those of other Lymnaeoidea possibly representing euthyneuran plesiomorphies. The Acroloxidae may therefore be sister to the remaining Lymnaeoidea, branching off before these gained their characteristic sperm traits.