Bacteria indigenous to a trichloroethylene (TCE) contaminated site were exposed to 0.56 and 6.7 mg L‐1TCE. Under the aerobic, oligotrophic conditions of the microcosms, the populations were able to degrade TCE and to increase their numbers by two orders of magnitude within one week. No other substrate was added. The average TCE disappearances were 47 percent and 33 percent for initial TCE concentrations of 0.56 and 6.7 mg L‐1, respectively. No TCE disappearance was observed in controls. Results of individual microcosms suggest that bacteria degraded TCE to levels as low as 1–2 percent of the initial concentration. Lag times of 14 and 18 days were observed for the 0.56 and 6.7 mg L‐1concentration microcosms, respectively. Bacterial population shifts were noted throughout the experiment. None of the isomers of a dehalogenation reaction were found.