Lab microcosm tests of the rate of toluene biodegradation were performed using soil from the A, B, and C horizons of the unsaturated zone of a sandy field site. Most of the soil samples had been previously exposed to toluene. Toluene biodegradation was rapid, occurring at a time scale comparable to the rate of sorption in many of the microcosms and demonstrating the potential for bioremediation of these contaminants in unsaturated soil. In the A horizon, with an initial toluene concentration in the solution phase of 4.5 mg/L, degradation was controlled by substrate-limited growth on toluene as the primary substrate. Soil from the B and C horizons initially showed similar behavior with a lower toluene concentration of about 2.5 mg/L.The maximum utilization rate (pmax) for soil from all three depths was 2.0 d-' . With repeated exposure to moderate to high concentrations of toluene, transformation in the B-and C-horizon soil appeared to be zero order, at a rate of 1 .O to 2.0 pg toluene/g soil/d. In C-horizon soil that had been taken directly from the field, the transformation rate was almost immeasurably slow. Addition of nitrogen as either ammonium or nitrate accelerated the degradation, showing that nitrogen was the most limiting nutrient.The apparent adaptation period observed before rapid toluene removal was fit by a substrate-limited growth model. Greater numbers of toluene-degrading microorganisms were found in soil exposed to toluene than in unexposed soil, supporting biomass growth as the explanation for the adaptation period. The results of enumeration of heterotrophs compared to the numbers of toluene degraders suggested that a small proportion of the total viable microorganisms were responsible for degradation of toluene.