Dehalococcoides bacteria that produce catabolic vinyl chloride (VC) reductive dehalogenase enzymes have been implicated as a requirement for successful biological dechlorination of VC to ethene in groundwater systems. Therefore, the functional genes in Dehalococcoides that produce VC reductase (e.g., vcrA) may be important biomarkers for predicting and monitoring the performance of bioremediation systems treating chloroethenes via enhanced reductive dechlorination (ERD). As part of an ERD field demonstration, 45 groundwater samples were analyzed for vcrA using quantitative PCR. The demonstration delivered lactate continuously via groundwater recirculation over 201 days to an aquifer contaminated with cis-1,2-dichloroethene (cDCE, similar to 150 mu M) and VC (similar to 80 mu M). Ethene (similar to 4 mu M) and Dehalococcoides containing vcrA (average concentration of 4 x 10(3) gene copies L-1) were detected a priori in the demonstration plot however, aquifer materials in a bench treatability test were able to dechlorinate cDCE with only a 4-month lag period. Given the short (7-month) schedule for the field demonstration, the field plot was bioaugmented on Day 69 with a mixed culture (KB-1) that included Dehalococcoides containing vcrA. Stimulated ethene generation commenced within four weeks of donor addition. Ethene concentrations increased until Day 145, and reached maximum concentrations of 10-25 mu M. Concentrations of vcrA increased concurrently with ethene production until Day 145, and plateaued thereafter at 10(7) to 10(8) gene copies L-1. These results indicate simultaneous growth of Dehalococcoides containing vcrA and ethene generation in an ERD field application. The quantitative increase in concentrations of Dehalococcoides containing vcrA at this site provide further evidence that the vcrA gene is an effective biomarker for field-scale ERD systems.