The spermatozoa of the gymnolaematous bryozoanElectra pilosaare thread-like cells about 350 μm long. The head region is needle-shaped, consisting of a long thread-like nucleus tapering anteriorly. No acrosorne is present. The midpiece consists of an axoneme surrounded by two long mitochondrial derivatives and two thin, nonmembrane-bounded granular rods alternating with the mitochondria. The two mitochondrial derivatives are of unequal size. The larger one is provided with lamellae of alternating electron-dense and electron-transparent material. Both mitochondria have a crescent form when seen in transverse section. In the tail region the axoneme is surrounded by a tube-like sheath composed of a thick cell membrane. An annulus is located at the junction between midpiece and tail. The flagellum is of the 9+2 type and in the tail region the number of microtubules is reduced. The nine peripheral components of the axoneme continue as single microtubules. Spermatozoaof Electraare tightly arranged in unencapsulated aggregates of c. 30 (32) cells capable of active swimming. Sperm heads and midpiece regions have a regular parallel orientation in the spermatozeugmata and are probably connected with each other by mucus. The presence of spermatozeugmata is indicative of internal fertilization inElectra.Ent-aquasperm of the type found inElectra pilosaare probably the most common type of gymnolaemate sperm. Sperm structure supports the hypothesis that Stenolaemata and Gymnolaemata are more closely related with each other than either of them is with the Phylactolaemata.