Quantitative and qualitative ultrastructural data from the spermatozoa of 11 oligochaete species, representing all orders (Tubificida, Lumbriculida, and Haplotaxida, yielded a single most parsimonious tree, using the Wagner -tree (PAUP) method of Swofford (1984). The Haplotaxida (Haplotaxis through Megascolecidae) form a discrete monophyletic group, with  inclusion, however, of the reproductively exceptional tubificidan Phreodrilus, Bythonomus, representing the Lumbriculidae, the phylogenetic position of which has been so controversial, forms the plesiomorphic sister group of the Haplotaxida, a position which is supported from other evidence. The Tubificida, represented by two tubificids (Rhizodrilus and Limnodriloides) and by the enchytraeid Lumbricillus, are plesiomorphic relative to the lumbriculid+haplotaxid assemblage and lie at the base of the tree, but all three appear mutually paraphyletic. Monophyly of the Tubificida cannot, however, be considered conclusively refuted from the small sample used. Lumbricillus appears to have the most pleisomorphic sperm in the investigated oligochaetes. The Megascolecidae (Amynthas and Fletcherodrilus) form the highest affinity and most apomorphic group. The  implications of relative apomorphy of the Lumbriculida (Bythonomus) are profound. Branchiobdellids and leeches are generally regarded as sharing a common ancestry with lumbriculids and would, because of the revised position of the latter, cladistically constitute part of the Oligochaeta sensu lato.