Trichloroethene is degraded by ammonia-oxidizing bacteria. In the absence of ammonia, TCE transformation proceeds at moderate rates. Under the experimental conditions tested, the presence of PCE and cis-DCE, competitive inhibitors of TCE, slowed the rate and extent of TCE transformation. The capacity of these microorganisms to degrade TCE in the absence or presence of mixtures is diminished by the inactivation of the cells as a consequence of the transformation of the chlorinated compounds. The potential application of ammonia oxidizers as a stand-alone in situ treatment process for TCE degradation is diminished by the inactivation of the cells and the inhibitory effects that mixtures of TCE and other chlorinated compounds pose to the microorganisms.