The effects of toluene on indigenous microbial populations involved in the soil nitrogen cycle were examined. Ammonia oxidation potential (AOP) and nitrite oxidation potential (NOP) were both reduced after incubation with high toluene concentrations for 45 days, with the former activity showing greater sensitivity. KCl-extractable ammonium (NH4+ext) levels increased dramatically in soil exposed to high toluene levels, and arginine ammonification was not significantly affected. Alfalfa-amended soil incubated in the presence of 200 txg toluene m1-1 showed progressive accumulation of NH4+eK~ over 37 days, indicating that mineralization of plant-associated nitrogen was not hindered by toluene. AOP in treated soil was much less than in control soil on days 7 and 37, but the MPN of ammonia oxidizers in control and exposed soil were not significantly different. Soil incubated with 100 rxg toluene m1-1 for 28 days, vented and allowed to incubate for an additional 7 to 30 days, exhibited only slight increases in AOP and NOE while NH4+e×t returned to control levels within a week. Soil exposed to 200 txg toluene m1-1 and treated in the same manner showed no increases in either AOP or NOP, and NH4+e~t remained elevated for the duration of the experiment, indicating more longterm effects on soil nitrogen cycling had occurred. Ammonia oxidizer levels in control soil and soil incubated with 100 Ixg toluene ml -~ were not appreciably different, whereas levels of ammonia oxidizers were very low in soil exposed to 200 txg toluene ml -~ and increased only slightly by 30 days post vent. Experiments to determine how toluene affects the AOP of soil indicated a competitive inhibition mechanism, with an effective concentration causing 50% reduction in activity (ECs0) of 11 tXM toluene, and a competitive inhibition constant (K0 of 0.1 _+ 0.05 IXM toluene. These results indicate the potential