An electron microscopic study of sperm and spermiogenesis revealed the process of formation of the monoaxonemal sperm inBaltoplana magna.Two basal bodies are present in the zone of differentiation of the young spermatid, on either side of an intercentriolar body, but only one develops into a normal flagellum. The other remains as a flagellum bud that disappears after the spermatid has become elongated and the normal flagellum has fused with the spermatid shaft in a distal to proximal direction. The mature sperm has a single incorporated axoneme of the trepaxonematan 9+“1” configuration, a full ring of longitudinal cortical microtubules, an elongate mitochondrial rod and an elongate nucleus with a number of dense longitudinal chromatin rods. There are no dense bodies present in the spermatids nor in any region of mature sperm.