Oxygen-sensitive Dehalococcoides bacteria play crucial roles in detoxification of chlorinated contaminants (e.g., chlorinated ethenes), and bioremediation monitoring relies on quantification of Dehalococcoides DNA and RNA biomarkers. To explore the effects of oxygen on Dehalococcoides activity, viability, and biomarker quantification, batch experiments with a tetrachloroethene-to-ethene dechlorinating consortium (Bio-Dechlor INOCULUM [BDI]) harboring multiple Dehalococcoides strains were performed to quantify the effects of < or = 4 mg/L dissolved oxygen. Oxygen inhibited reductive dechlorination, and only incomplete dechlorination to vinyl chloride (VC) occurred following oxygen consumption and extended incubation periods (89 days). Following 30 days of oxygen exposure and subsequent oxygen removal (i.e., reversibility experiments), all trichloroethene- (TCE-) fed cultures dechlorinated TCE to VC, but VC dechlorination to ethene occurred in only one out of fourteen replicates. These results suggest that Dehalococcoides strains respond differently to oxygen exposure, and strains catalyzing the VC-to-ethene dechlorination step are more susceptible to oxygen inhibition. Quantitative real-time PCR (qPCR) analysis detected a 1-1.5 order-of-magnitude decrease in the number of Dehalococcoides biomarker genes (i.e., 16S rRNA gene and the reductive dehalogenase [RDase] genes tceA, vcrA, bvcA) in the oxygen-amended cultures, but qPCR analysis failed to distinguish viable, dechlorinating from irreversibly inhibited (nonviable) Dehalococcoides cells. Reverse transcriptase qPCR (RT-qPCR) detected Dehalococcoides gene transcripts in the oxygen-amended, non-dechlorinating cultures, and biomarker transcription did not always correlate with dechlorination (in)activity. Enhanced molecular tools that complement existing protocols and provide quantitative information on the viability and activity of the Dehalococcoides population are desirable.