Summary Using transmission electron microscopy, sperm morphology was investigated for the first time in a representative of the heterodont bivalve family Glauconomidae. Spermatozoa of Glauconome plankta (Iredale, 1936) consist of a head region (acrosomal complex + nucleus), short midpiece and a flagellum. The membrane-bound acrosomal vesicle is attenuate-conical (length 1.1 ± 0.08 μm) with a moderately deep basal invagination extending for most of the vesicle's length. Contents of the vesicle are differentiated into a highly electron-dense basal ring (cylindrical) sheathed by less dense material that also occupies the anterior two-thirds of the vesicle. Finely granular subacrosomal material fills the vesicle invagination and forms a layer between the nuclear apex and the vesicle, but no axial rod is discernible. The nucleus is rodshaped and straight, 4.4 ± 0.2 μm long and tapers slightly towards the apex. Nuclear contents are highly electron-dense, with the exception of occasional, electron-lucent lacunae. The midpiece consists of a pair of triplet-microtubular centrioles surrounded by four, approximately spherical mitochondria. Highly electron-dense granules of putative glycogen were observed between the mitochondria and around the distal centriole. The distal centriole is anchored to the plasma membrane via satellite fibres and is continuous with the flagellum (the latter 46 ± 3.0 μm long and exhibiting a conventional 9+2 axoneme). Comparison of sperm results does not suggest any especially close relationship between Glauconomidae (as exemplified by G. plankta) and other member families of the superfamily Veneroidea. Elsewhere within the Heterodonta no close relationship with the Corbiculoidea, Myoidea, Mactroidea, Tellinoidea or the Crassatelloidea is indicated by sperm morphology. Most promising seems a possible connection with Arctica (Arcticidae, Arcticoidea), which shows similar acrosomal and nuclear features. Supporting morphological and molecular evidence for this relationship is inconclusive but suggests further study.