Galeolaria caespitosa is a native tube dwelling marine invertebrate which broadly distributes along the intertidal zone of South-eastern Australian coast. It is a broadcast-spawning species whose abdomen is filled up with viable gametes throughout the year. The pattern of spermatogenesis was investigated at the ultrastructural level using both transmission and scanning electron microscopy. The proliferation of spermatogonia occurred in cell clusters attached to the ciliated intersegmental peritoneal septum, forming a germinal center located at the dorsal region of each segment. Germ cells migrated peripherally while undergoing the first meiotic division; the formation of synaptonemal complexes being detected in primary spermatocytes. Paired secondary spermatocytes detached from cell clusters in the germinal center and completed the remainder of spermatogenesis in coelomic fluid. Tetrads of round spermatids were generated as a result of the second meiotic division. The acrosome first appeared in the posterior end of the cytoplasm as a spherical proacrosomal vesicle with an associated Golgi complex. The proacrosome migrated toward the anterior end of the sperm during spermiogenesis and reached the apex prior to the completion of nuclear condensation. The mature spermatozoa were of the ect-aquasperm type, with a cap-like acrosome, an oval nucleus, a midpiece containing four round mitochondria and a flagellum with a typical 9 + 2 configuration. During maturation spermatogenic cells migrated ventrally away from the germinal center in each segment, and were eventually released into the water column from the main sperm duct, which was longitudinally distributed in the ventral region of the abdomen. Since a portion of spermatogenesis in this species involves cells floating freely in the coelomic cavity, secondary spermatocytes have been successfully purified on Percoll gradients, cultured in RPMI medium 1640 and shown to complete the entire process of spermiogenesis in vitro.