Field and laboratory studies were conducted to investigate the degradation of the organophosphorus insecticide isazofos (0-5-chloro-l-isopropyl-1H-l,2,4-triazol-3-ylO,O-diethyl phosphorothioate) in soil. In a 6-year field study, soil pH was an important factor influencing the degradation of isazofos inasmuch as an increased rate of degradation was observed in soils with previous isazofos applications and pH of 6.9 or more. A laboratory study of [14Clisazofos confirmed the rapid degradation of this insecticide in high pH soils. No increased degradation rate, however, was observed in sterilized high-pH soils. The availability of isazofos to microorganisms, based on sorption of isazofos to soil, seems to be an important factor influencing the degradation of isazofos in soil. Sorption coefficients were negatively correlated with isazofos degradation rate.