The strontium : calcium concentration ratios in the otoliths of ten variously-sized Tribolodon nakamurai (Cyprinidae) individuals collected from the lower reaches of the Mogami River were examined and compared with those of six spawning individuals from the upper reaches of the river in order to establish the existence or otherwise of any sea-migrating traits. The ratios were relatively stable from the core to the edge of the otolith in all specimens, indicating a solely freshwater life style. Any conservation program for T. nakamurai should therefore be based on the premise that the species has a fluvial mode of life.