A decline in the brown planthopper control effectivity of diazinon was noticed after its continuous application to rice paddies for [Formula: see text] years at the International Rice Research Institute. In a study undertaken to determine the factor involved in this decline, rapid inactivation of diazinon was recorded within 3 to 5 days of its incubation with water from a rice field that had received several applications of diazinon. During the same period, its degradation in water from an untreated rice field was non-significant. This indicated the development of a degrading agent in paddy water following diazinon treatments.The inactivation of diazinon on incubation with water from treated fields was retarded if the incubation mixture was sterilized or kept anaerobically. Release of 14CO2 from 14C-diazinon was rapid from water of treated fields, but was inhibited when streptomycin was added to the system. A lag of 2 or 3 days, followed by a rapid degradation, was generally evident. Thus these data established the fact that aerobic biological agents, capable of degrading diazinon in paddy water, develop after diazinon applications. A bacterium, Arthrobacter sp., capable of metabolizing diazinon in the presence of ethyl alcohol or glucose was isolated from paddy water of treated fields.