Fossil rodents, represented by nine isolated teeth, were obtained by wet sieving sediments from the Upper Pliocene Tsubusagawa Formation in Ajimu, Oita Prefecture. The rodent remains include Micromys sp., Arvicolinae gen. et sp. indet., and unidentified incisors. This is the first report of the Pliocene rodent fauna in Japan. Micromys sp. fossils differ from modern species in occlusal outline, number of roots, crown size, position of t4 and t6, size of t9, and presence or absence of t7 in the upper M1. Arvicorinae gen. et sp. indet. have such features as a hypsodont, very high sinuous line, thick cementum, and relatively wide crown in comparison to other arvicolids. Historically, Micromys and Arvicolinae have had a Palearctic distribution. This is consistent with previous plant fossil research that indicates a temperate-zone climate in the lower to middle part of the Tsubusagawa Formation. However, it is not consistent with the fossil tortoise and large mammals included in the Ajimu Fauna, which represent a subtropical or tropical This contradiction may suggest that the fossil assemblage was formed in an environment under the transition from tropical to temperate zone in the gradually cooling climate of the Late Pliocene.
