Genetic divergence in Ditrema jordani was investigated from sequence variations on the mitochondrial cytochrome b gene. Clear genetic differences were found between specimens collected from the Mie and Shizuoka prefectures (westward of Izu Peninsula) and those from the Kanagawa and Chiba prefectures (eastward of Izu Peninsula). The uncorrected genetic distance between the two groups, which may represent separate taxa, was much greater (3.1-3.7%) than that between D. temminckii and D. viride (1.1-2.4%), and between two subspecies of D. temminckii (0.8-1.3%), suggesting that the Izu Peninsula acts as a stable geographic barrier to gene flow between the D. jordani groups. The clear genetic divergence between the two geographic populations of D. jordani may be partly related to direct development (viviparity) and low dispersal ability in the genus.