The effects of trichloroethylene (TCE) and toluene on indigenous soil communities were examined in soil microcosms, with special emphasis on community structure and function. Soil communities were exposed to TCE (1, 30 and 60 μg ml⁻¹) plus toluene (20 μg ml⁻¹), TCE only (1, 30 and 60 μg ml⁻¹), and toluene only (20, 60, 100 and 200 μg ml⁻¹) (all concentrations given as μg ml⁻¹ soil solution after taking into account sorption and volatilization). Soil microbial populations exposed to 30 or 60 μg TCE ml⁻¹ plus 20 μg toluene ml⁻¹, or to toluene concentrations greater than 60 μg toluene ml⁻¹, exhibited different sole carbon source utilization patterns than control soils, as revealed by Biolog Gram negative (GN) plates. These differences remained even 30 days after all the TCE and toluene had been removed from the system. Exposure to TCE alone did not alter Biolog patterns to the same degree as TCE combined with toluene. Thirty and 60 μg TCE ml⁻¹, either in the presence or absence of 20 μg toluene ml⁻¹, slightly reduced basal respiration after 28 days of incubation, whereas substrate induced respiration was not affected. Total culturable heterotrophs were not sensitive to TCE and toluene, and were only slightly reduced after exposure to 200 μg ml⁻¹ toluene for 45 days. Most protozoa were insensitive to TCE and toluene, but TCE was toxic to flagellates. Significant decreases in numbers of nematode types occurred after exposure to 30 or 60 μg TCE ml⁻¹. The nematode maturity index (MI), an index of diversity based on trophic groupings, was not sensitive to TCE or to TCE combined with toluene in an agricultural soil, whereas the MI in a riparian soil was reduced by concentrations of 30 μg TCE ml⁻¹. These results indicate that TCE and toluene adversely affect the biota of the soil ecosystem, sometimes for prolonged periods.