This study obtained 18S rDNA sequence data to investigate the extent of genetic divergence in the Japanese dendrocoelid group, and applied the data to the estimation of phylogenetic relationships and taxonomy. The sequence data, consisting of 558 base positions in an aligned data set, were obtained by using the polymerase chain reaction and direct sequencing. For comparative analysis, the 18S rRNA gene sequence data on dendrocoelids from different distant geographical places (Europe, Lake Baikal in Siberia, Kamchatka) published previously were used. Such analysis showed that: (1) Bdellocephala species from Japan, Kamchatka and Lake Baikal are closely related ; (2) Japanese species classified within the genus Dendrocoelopsis do not form a single cluster of closely related organisms; (3) Dendrocoelopsis ichikawai differs minimally from Bdellocephala baicalensis (from Lake Baikal), reliably groups with all Bdellocephala, and should therefore be classed with this genus.

Taxonomic conclusions. (1) The genus Dendrocoelopsis should be revised after molecular typification of European and American representatives. (2) The current generic status of D. ichikawai based on the presence of a well-developed penis papilla should be reconsidered. (3) Sequence divergence amongst representatives of B. brunnea indicates that representatives should be closely examined for the presence of morphological characters by which they might be distinguished.