We conducted experiments on the secondary seed dispersal of walnut trees (Juglans ailanthifolia) by rodents for two years and surveyed the distribution of walnut trees in a riparian forest on Higashiootori river, Yamagata, northern Japan. We placed 1, 500 marked nuts in the center of each of three 50 m×50 m plots. All nuts disappeared in a month in both years. We recovered 7.1% of the marked nuts, 96% of which were eaten by mice (Apodemus speciosus) or squirrels (Sciurus lis). Twenty-six seedlings with marked nuts were found in the three plots, and one of them germinated in the summer three years after the seed placement. Seedlings germinated from the nuts scatter-hoarded by rodents and were dispersed widely regardless of their relative height above water level. Walnut trees within plots were also distributed independently of relative height. These results suggest that most of the fallen nuts were transported and buried by rodents, and consequently that few nuts were swept away by overflow in snowmelt seasons. Scatter-hoarding behavior of rodents may contribute to the regeneration of walnut trees in riparian forests.