Stable carbon isotopic analysis has the potential to assess biodegradation of chlorinated ethenes. Significant isotopic shifts, which can be described by Rayleigh enrichment factors, have been observed for the biodegradation of trichloroethlyene (TCE), cis-dichloroethylene (cDCE), and vinyl chloride (VC). However, until this time, no systematic investigation of isotopic fractionation during perchloroethylene (PCE) degradation has been undertaken. In addition, there has been no comparison of isotopic fractionation by different microbial consortia, nor has there been a comparison of isotopic fractionation by consortia generated from the same source, but growing under different conditions. This study characterized carbon isotopic fractionation during reductive dechlorination of the chlorinated ethenes, PCE in particular, for microbial consortia from two different sources growing under different environmental conditions in order to assess the extent to which different microbial consortia result in different fractionation factors. Rayleigh enrichment factors of -13.8@1000, -20.4@1000, and -22.4@1000 were observed for TCE, cDCE, and VC, respectively, for dechlorination by the KB-1 consortium. In contrast, isotopic fractionation during reductive dechlorination of perchloroethylene (PCE) could not always be approximated by a Rayleigh model. Dechlorination by one consortium followed Rayleigh behavior (epsilon = -5.2), while a systematic change in the enrichment factor was observed over the course of PCE degradation by two other consortia. Comparison of all reported enrichment factors for reductive dechlorination of the chlorinated ethenes shows significant variation between experiments. Despite this variability, these results demonstrate that carbon isotopic analysis can provide qualitative evidence of the occurrence and relative extent of microbial reductive dechlorination of the chlorinated ethenes.