The reductive dechlorination of pentachlorophenol (PCP) was investigated in anaerobic sediments that contained nonadapted or 2,4- or 3,4-dichlorophenol-adapted microbial communities. Adaptation of sediment communities increased the rate of conversion of 2,4- or 3,4-DCP to monochlorophenols (CPs) and eliminated the lag phase before dechlorination was observed. Both 2,4- and 3,4-DCP-adapted sediment communities dechlorinated the six DCP isomers to CPs. The specificity of chlorine removal from the DCP isomers indicated a preference for ortho-chlorine removal by 2,4-DCP-adapted sediment communities and for para-chlorine removal by 3,4-DCP-adapted sediment communities. Sediment slurries containing nonadapted microbial communities either did not dechlorinate PCP or did so following a lag phase of at least 40 days. Sediment communities adapted to dechlorinate 2,4- or 3,4-DCP dechlorinated PCP without PCP without an initial lag phase. The 2,4-DCP-adapted communities initially removed the ortho-chlorine from PCP, whereas the 3,4-DCP-adapted communities initially removed the para-chlorine from PCP. A 1:1 mixture of the adapted sediment communities also dechlorinated PCP without a lag phase. Intermediate products of degradation--2,3,5,6-tetrachlorophenol, 2,3,5-trichlorophenol, 3,5-DCP, 3-CP, and phenol--were identified by a combination of cochromatography (high-pressure liquid chromatography) with standards and gas chromatography-mass spectrometry. (Copyright (c) 1991, American Society for Microbiology.)