Abstract: Degradation characteristics of O phosphothioate insecticides by the isolated strain Pseudomonas sp. YF05 were measured. Degradation of parathion was determined to be first order kinetic under conditions of pH7.0, 30 ℃, and biomass of OD 415 nm 0.2. The rate constants for degradation of parathion at concentrations of 10, 25, 50, 100, 200, 500 mg/L, were measured to be 0.077, 0.123, 0.128, 0.119, 0.084, 0.013, respectively. The capability of YF05 was inhibited by parathion at high concentration. The degradation rate of parathion by YF05 was affected by pH and temperature as the orders of pH 8.0>pH 7.0>pH 6.0, 40 ℃> 30 ℃ >20 ℃, respectively. Parathion-methyl and fenitrothion could be degraded rapidly by the isolated strain YF05. The rate constant for degradation of parathion methyl was almost the same as that of parathion, and that of fenitrothion was greater than that of parathion.