The influence of dechlorinating microorganisms on PCE and its reduction products in a residual non-aqueous phase liquid (NAPL) source zone was investigated. Experiments were conducted in upflow anaerobic columns containing glass beads contaminated with a residual NAPL consisting of tridecane and PCE. Columns were inoculated with a mixed PCE-dechlorinating culture, and fed a nutrient medium that included electron donor (pyruvate and/or lactate) at a wide range of concentrations. The systems that were monitored for effluent organic acids (pyruvate-fed) demonstrated evidence of active fermentation. Comparisons between biotic and abiotic columns demonstrated that dechlorination resulted in an increase in total PCE removal, up to a factor of 16 over dissolution. PCE was sequentially reduced as far as vinyl chloride. Total chlorinated ethenes removal was enhanced up to 6.5 times over dissolution removal. The system fed the highest electron donor level, interestingly, lost dechlorinating activity early in the experiment.