cis-1,2-Dichloroethylene (cis-DCE) is frequently found at significant concentrations in groundwater which is contaminated with tetrachloroethylene or trichloroethylene. Under anaerobic conditions, cis-DCE can be biotransformed via reductive dechlorination to ethylene. Several factors affecting this transformation were investigated using anaerobic sewage sludge as an inoculum. The reductive dechlorination of cis-DCE was observed at 25°C and 15°C but not at 35°C. Supplying a suitable electron donor (organic substrate or hydrogen) was necessary to sustain reductive dechlorination. Glucose, yeast extract, propionate, and hydrogen stimulated dechlorination, while methanol and acetate did not. Anaerobic enrichment cultures capable of dechlorinating cis-DCE to ethylene were developed from the sludge. In the presence of either glucose, yeast extract or propionate (100 mgCOD/l), 0.46 mg/l of cis-DCE was almost completely dechlorinated to ethylene within 4 days by the cultures at 25°C. Transformation rate was somewhat lower in the culture fed with hydrogen. Dechlorinating ability was sustained even in the cultures fed with low concentrations (10 mgCOD/l) of glucose or hydrogen, although the transformation was sometimes insufficient. These results suggest that anaerobic bioremediation processes can be used for removal of chlorinated ethylenes from contaminated groundwater.