AbstractPast studies have shown that extremely high concentrations of alachlor in soil can depress microbial biomass and bioactivity; consequently, degradation of alachlor is also inhibited. Studies were undertaken to further characterize the relationship among alachlor concentration in soil, dehydrogenase activity (as an indicator of microbial activity), and degradation rate. Alachlor initially inhibited soil dehydrogenase in soil at concentrations as low as 250 mg/kg with prolonged inhibition through at least 21 d occurring at concentrations ≥750 mg/kg. The inhibition of soil dehydrogenase was associated with a prolonged persistence of alachlor beyond the previously reported ranges of half‐life observed at normal field rates of application. Amendment of soil with cornmeal caused degradation of 10 to 250 mg/kg alachlor at rates substantially faster than previously reported for laboratory incubations. At alachlor concentrations ≥750 mg/kg, dehydrogenase activities in amended soils surpassed levels in corresponding no‐pesticide controls after 21 d; coincidentally,>50% of the initially added alachlor had degraded during the same period. These results suggested that stimulation of microbial bioactivity by addition of organic amendments may enhance co‐metabolism of high concentrations of pesticide