We report measurements of the C and N stable isotope ratios (δ13C and δ15N) in tissue of the bivalve Corbicula japonica and particulate organic matter (POM), and the C/N ratios of microalgae, terrestrial plants and sedimentary organic matter in Natori River, Miyagi Prefecture, Japan. δ13C of the bivalve and POM at upstream stations (4.0∼4.5 km from the river mouth) were significantly lower than those in the downstream station (1.5 km from the river mouth). The low δ13C values in the upper two stations (between - 28.0 and - 25.5%) indicated that terrestrial organic matter is the primary C source of the bivalve. However, δ15N values for the bivalve at all stations (between 9.5 and 10.8%) did not dišer significantly. This suggests that benthic microalgae are the primary N source in all stations. We conclude that C and N sources for the bivalve originate from different food items in the upper reaches of the river.