Concentrations of cesium isotopes and plutonium in river water samples in Japan, collected during the period from June 1985 to February 1987, have been measured. The total137Cs concentrations in the Japanese river waters ranged from 0.063 to 1.89 mBq·l−1. The portion of particulate137Cs to total was observed to be less than 10 to 35%. The total239,240Pu concentrations ranged from 0.56 to 1.93 μBq·l−1. Particulate239,240Pu occupied 13 to 95% of the total. After the Chernobyl fallout, elevated137Cs concentrations were observed in the Japanese river waters as well as the detection of134Cs, whereas there was no effect on the river plutonium from the Chernobyl fallout. The partition coefficients of137Cs and plutonium between suspended particulate and dissolved phases in the Japanese rivers were determined: from 1.0·104 to 3.2·105 and from 4.1·104 to 2.3·106 for137Cs and plutonium, respectively. The result suggests that these radionuclides, especially plutonium, are tightly associated with soil particles and/or suspended matter.