Tetrachloroethene (PCE)- and trichloroethene (TCE)-impacted sites pose significant challenges even when site characterization activities indicate that biodegradation has occurred naturally. Although site-specific, regulatory, and economic factors play roles in the remedy-selection process, the application of molecular biological tools to the bioremediation field has streamlined the assessment of remedial alternatives and allowed for detailed evaluation of the chosen remedial technology. The case study described here was performed at a PCE-impacted site at which reductive dechlorination of PCE and TCE had led to accumulation of cis-dichlorethene (cis-DCE) with concentrations ranging from approximately 10 to 100 mg/L. Bio-Trap® samplers and quantitative polymerase chain reaction (qPCR) enumeration of Dehalococcoides spp. were used to evaluate three remedial options: monitored natural attenuation, biostimulation with HRC®, and biostimulation with HRC-S®. Dehalococcoides populations in HRC-S-amended Bio-Traps deployed in impacted wells were on the order of 103 to 104 cells/bead but were below detection limits in most unamended and HRC-amended Bio-Traps. Thus the in situ Bio-Trap study identified biostimulation with HRC-S as the recommended approach, which was further evaluated with a pilot study. After the pilot HRC-S injection, Dehalococcoides populations increased to 106 to 107 cells/bead, and concentrations of cis-DCE and vinyl chloride decreased with concurrent ethene production. Based on these results, a full-scale HRC-S injection was designed and implemented at the site. As with the pilot study, full-scale HRC-S injection promoted growth of Dehalococcoides spp. and stimulated reductive dechlorination of the daughter products cis-DCE and vinyl chloride. © 2008 Wiley Periodicals, Inc.