The soybean plants in monoculture plots in Hokkaido Prefectural Kitami Agricultural Experiment Station showed severe necrosis of roots, and their growth was retarded. Pythium myriotylum and Pythium sp., pathogenic to soybean plants, were isolated only from the necrotic root tissues in monoculture. When ridomil, N-(2, 6-dimethylphenyl)-N-(methoxyacetyl) alanine methyl ester, was drenched to the soil collected from the monoculture plots, the roots of soybean seedlings grown in the soil remained healthy. These results suggest that the symptoms of soybean plants in monoculture are caused by the infections of P. myriotylum and Pythium sp.