Anaerobic sludge granules from two different sources were maintained in serum bottles and fed at 3-d intervals in the presence of 0.2 ppm perchloroethylene (PCE). Following acclimation periods ranging from 48-79 d, PCE degradation was observed with 95% utilization in 3 d. Only granules amended with formate as a cosubstrate showed PCE-degrading activity, whereas those utilizing acetate, lactate, ethanol, and methanol remained PCE-inactive after 90 d. Both cosubstrate consumption and PCE degradation ceased when acetate replaced formate as the cosubstrate for formate-acclimated cultures. The total moles of chlorinated and nonchlorinated ethene moieties were found to remain constant over time, demonstrating conservation of mass. Trichloroethylene (TCE) and cis-1,2 dichloroethylene (cis-1,2 DCE) were identified as the major dechlorination end products. The absence of further dechlorination in the presence of a great excess of reducing equivalents (13,000 times that needed) supports the hypothesis that complete degradation of PCE to ethene is not solely dependent on excess reducing equivalents, but also requires the presence of an appropriate microbial consortium.