Spermatozoa of the basommatophoran pulmonate families Ellobiidae (Ophicardelus ornatusFerussac), Amphibolidae (Salinator fragilisLamarck,Salinator solidavon Martens), Siphonariidae (Siphonaria funiculataReeve) and Lymnaeidae (Lymnaea lessoni(Deshayes)) were studied using transmission electron microscopy. Spermatozoa of all species, like most euthyneuran spermatozoa, possess (1) an acrosome composed of an apical vesicle and acrosomal pedestal (many differences between families), (2) a helically keeled, posteriorly invaginated nucleus, (3) a midpiece composed of paracrystalline and matrix materials and a variable number of incorporated glycogen‐filled helices (one inSalinatorandSiphonaria, two or three inLymnaea, three inOphicardelus) and (4) an axoneme associated with coarse fibres (periodically banded in “neck” region) and rows of intra‐axonemal granules. The wide diversity of spermatozoon structure in the species studied, in particular the midpiece structure ofSiphonaria(which resembles closely that of certain opisthobranchs) indicates that the Basommatophora may not represent a valid taxonomic unit. A comparison of basommatophoran sperm with other euthyneurans and with euspermatozoa of prosobranchs i