Three pesticides for paddy rice, iprobenfos, isoprothiolane, and diazinon were examined on some environmental factors, their hydrolysis, microbial degradation, and photolysis in aqueous systems. Iprobenfos was mainly degraded by microorganisms and its half-life was 5.7 days at $28^{circ}C$ in aqueous systems. Hydrolysis of iprobenfos was accelerated by the higher temperature, but its photodegradation was accelerated by the lower pH. Isoprothiolane was rapidly decomposed by two factors, microorganisms and sunlight. The half-life of isoprothiolane by sunlight was 91 days at pH 9.0, while it was 13 days at pH 4.0 and 16 days at pH 7.2. However, it was shortened under low pH condition. In aqueous system, diazinon was degraded by all of three factors and its degradation rate was remarkably accelerated by acidic solution. Main degradation factors of iprobenfos, isoprothiolane, and diazinon in the aqueous system were investigated by microbial degradation, photolysis, and hydrolysis, respectively. The strains of microbial degradation for iprobenfos, isoprothiolane, and diazinon in the aqueous environment were identified as Pseudomonas putida, Alcaligenes xylosoxydans ss, Klebsiella planticola/ornithinllytica, respectively. The similarity rates of identity were $54.8{sim}86.2%$ with biolog-system.