The effects of soil environmental conditions on the rate of degradation of fungicide IBP (Iprobenfos, S-benzyl O, O-diisopropyl phosphorothioate) in the soils under flooded condintions were examined in the laboratory. IBP in soil was degraded more slowly under flooded conditions than under upland conditions. The degradation greatly varied among soils, and the degradation rate was negatively correlated with the content of soil organic matter. Degradation of IBP was influenced by the soil temperature and the amount of IBP applied. The rate of degradation in soil was remarkably inhibited by the amendment of rice straw but not affected by the treatment of mixed-fertilizer, and insecticide fenitrothion and herbicide butachlor. The degradation of IBP was assumed to be due to microorganisms, especially aerobic microbes, as no degradation was observed in sterilized soil.