Tetrachloroethylene (PCE) is thought to have no natural source, so it is one of the most difficult contaminants to degrade biologically. This common groundwater pollutant was thought completely nonbiodegradable in the presence of oxygen. Here we report that the wastewater bacterium Pseudomonas stutzeri OX1 degrades aerobically 0.56 μmol of 2.0 μmol PCE in 21 h (V(max) ≃ 2.5 nmol min mg protein and K(M) ≃ 34 μM). These results were corroborated by the generation of 0.48 μmol of the degradation product, chloride ions. This degradation was confirmed to be a result of expression of toluene-o-xylene monooxygenase (ToMO) by P. stutzeti OX1, since cloning and expressing this enzyme in Escherichia coli led to the aerobic degradation of 0.19 μmol of 2.0 μmol PCE and the generation of stoichiometric amounts of chloride. In addition, PCE induces formation of ToMO, which leads to its own degradation in P. stutzeri OX1. Degradation intermediates reduce the growth rate of this strain by 27%.