The ultrastructure of sperm of an acochlidian opisthobranch is described for the first time in detail, inthe tiny, gonochoristic Microhedyle remanei (Microhedylidae) from Bermuda. Transmission electron microscopy(TEM) shows the spermatozoa of M. remanei sharing many plesiomorphic features withopisthobranch gastropods, such as having an apical spiral nucleus with a basal invagination filled with abell-shaped centriolar derivative; there is a single glycogen helix embedded into matrix and paracrystallinematerial. Sperm of M. remanei are characterized by possessing a strongly helically coiled nucleus withup to five keels terminating into one very prominent and intertwined keel. The sperm midpiece shows theglycogen helix which is very densely arranged. This clearly differs from Hedylopsis ballantinei, a member ofthe related acochlidian family Hedylopsidae, where three glycogen helices with different lengths and aprobably much shorter nucleus are present. This variation among acochlidian sperm may be phylogeneticallyrelevant and may be related to special acochlidian reproductive features such as sperm transfer viahypodermic injection or dermal fertilization via spermatophores.