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Analyses using PAUP (phylogenetic analysis using parsimony) were conducted on a matrix of 24 taxa and 26 characters. The taxa included generalised descriptions of a haplotaxid and a lumbriculid oligochaete as outgroups with a hypothetical branchiobdellidan ancestor and 21 genera as ingroups. The branchiobdellidan taxon is a monophyly, and using anatomical characters only, is composed of three suprageneric assemblages. The taxonomic rank of the taxon and assemblages cannot be determined until they have been compared with cladistic analyses on other closely related clitellate groups. In the meantime the currently accepted ordinal rank of Branchiobdellida and the major branchiobdellidan families should continue to be used, with the latter reflecting the assemblages of the genera in our analyses: Branchiobdellidae (including Holtodrilus n.gen., Sinodrilus n.gen., and Xironodrilus), Bdellodrilidae (including Caridinophila and Hidejiodrilus n.gen.), and Cambarincolidae. An analysis with two zoogeographical characters in the matrix caused the genera to be divided into their respective North American and Eurasian groups; however, the North American Triannulata appeared constantly in the latter group. The genera in the Branchiobdellidae, as defined by us, are found in both geographical regions, and predictably the assemblage was split in two.

The ultrastructure of mature spermatozoa from five species of branchiobdellidans is described. The species come from four genera, and the three major families in the Branchiobdellida (Annelida: Clitellata) are all represented. The spermatozoa of Branchiobdella pentodonta and B. astaci (Branchiobdellidae) have a very long helical acrosome and a helical marginal fiber surrounding the flagellum, whereas those of Xironogiton instabilis show a twisted acrosome similar to that found in some oligochaetes. Bdellodrilus illuminatus (Bdellodrilidae) is extremely atypical, with no true acrosome, and with a long helical end-piece. Cambarincola fallax (Cambarincolidae) has an acrosome with a helical ridge on the acrosome tube, as in hirudineans, but shares with Branchiobdella spp. a helical marginal fiber around the flagellum. The spermatozoan characters in these species display a mixture of oligochaete and hirudinean features, as well as some autapomorphies, thus dispelling the idea that branchiobdellidan spermatozoa are very similar. Although a unique model cannot yet be defined for branchiobdellidan spermatozoa, a number of characters do show promise. These observations are still too few to allow any inference of evolutionary polarity in the Branchiobdellida or Clitellata, but they do suggest the possibility of using the comparative ultrastructure of the spermatozoa as an additional tool for studying branchiobdellidan phylogeny.