AbstractThe utility of sperm morphology in the systematics of polychaetes has been questioned in the past. This doubt stems from a problem with methodology, not with sperm morphology. It is argued that sperm characters used in combination with other morphological features have utility at various hierarchical levels. As a test of this proposition males in species from each of the following 10 fabriciin genera:Augeneriella, Fabricia, Fabriciola, Fahricinuda, Manayunkia, Novafabricia, Parafabricia, Pseudofabricia, Pseudofabriciolaand an undescribed new Genus A (Fitzhughet al., in preparation), were examined in order to provide new characters for phylogenetic systematic studies. All species were found to have a dorsal sperm duct running beneath the faecal groove of the thoracic region. No sabellin sabellids or serpulids have this duct. The above Fabriciinae have spermatids developing in large clusters of several hundred cells. Serpulids and sabellins nearly always have sperm developing in tetrads or small groups. Fabriciins studied also have a sperm structure distinct from the Sabellinae and the Serpulidae. Proposed synapomorphies for the members of the Fabriciinae, based on sperm structure, include (1) a thick glycocalyx over the plasma membrane; (2) a distinctive nuclear projection, with an anterior thickening of the nuclear membrane; (3) a thickened. spiraling ridge of nuclear membrane; (4) an extra‐axonemal sheath and (5) a unique sheath of mitochondrial material in the midpiece. Within the sub‐family there were differences among species in sperm structure. This variability involves acrosome morphology; the structure of the sperm nucleus and nuclear projection; the spiral of thickened nuclear membrane; the structure of the extra‐axonemal sheath; the mitochondrial sheath. The monophyly of the Fabriciinae (sensuFitzhugh, 1991, 1993) is well supported by this analysis. The status ofCaobangiais still unresolved and a close examination of this genus is warr