Simulated landfill gas, consisting of an equimolar mixture of carbon dioxide and methane and containing tetrachloroethylene (PCE) as a model impurity compound, was treated in a biofilter under anaerobic conditions. The PCE concentrations ranged from 30 to 100 ppm. Sucrose was added as an additional energy and carbon source. Tetrachloroethylene was dechlorinated completely. The final end-product of biodegradation could not be identified, but chlorinated hydrocarbons (vinyl chloride, dichloroethylene and trichloroethylene) were excluded. After an acclimation period the biofilter's PCE removal efficiency was higher than 98%. Granular activated carbon (GAC) was chosen as the support material for the biomass. GAC is a good sorbent, and PCE sorption probably played a major role in the successful biodegradation. The experiments, on the other hand, all indicate that tetrachloroethylene removal was due to metabolic degradation rather than sorption on the activated carbon. The removal rate was estimated to be 4.1 g PCE/(hm(3) biofilter volume). To the best of our knowledge, this is the first study reporting PCE biodegradation in an anaerobic, waste-gas environment.