The spermatozoa of clitellates are filiform cells with the acrosome situated in the anteriormost position, followed by the nucleus, the midpiece, and the tail. With respect to the basic plan of a 'modified′ sperm type, the clitellate spermatozoon is characterized by the presence of an acrosome tube containing to a variable extent the (withdrawn) acrosome vesicle, by the mitochondria being interposed between the nucleus and the tail, and by peculiar modifications of the central apparatus of the axoneme. The oligochaetes sensu stricto are characterized by the presence of a basal cylinder situated inside the basal body; the branchiobdellidans by an apical, conical indentation of the nucleus and by a helical marginal fiber coiled around the tail; the acanthobdellids by a dense sheath and accessory fibers surrounding the axoneme; the euhirudineans by an anterior prolongation of the acrosome tube, the anterior acrosome. Other sperm characters are shared by various combinations of taxa. With the aim of using sperm morphology as a tool for phylogenetic analysis, we identified 22 sperm characters in 21 species belonging to 12 clitellate families: Capilloventridae, Enchytraeidae, Tubificidae (with representatives of three subfamilies), Naididae, Lumbriculidae, Branchiobdellidae, Bdellodrilidae, Cambarincolidae, Acanthobdellidae, Piscicolidae, Erpobdellidae, Hirudinidae. By the assumption of the monophyly of those Clitellata having atria or derivatives thereof associated with the male ducts, and using one enchytraeid and one capilloventrid as outgroups, we analysed the data using the computer programme PAUP. Our results strongly support the monophyly of the hirudinean (i.e., acanthobdellid+euhirudinean) assemblage, as well as the close relationship between this group and the branchiobdellidans. Although a link between the branchiobdellidan-hirudinean clade and the Lumbriculidae is not refuted, sperm do not provide enough evidence for establishing the exact phylogenetic position of this clade within the Oligochaeta.