An anaerobic bacterium, strain PCP-1T (T = type strain), which dechlorinates pentachlorophenol (PCP) to 3-chlorophenol, was isolated from a methanogenic consortium. This organism is a spore-forming rod-shaped bacterium that is nonmotile, asaccharolytic, and Gram stain negative but Gram type positive as determined by electron microscopic observations. Inorganic electron acceptors, such as sulfite, thiosulfate, and nitrate (but not sulfate), stimulate growth in the presence of pyruvate and yeast extract. The optimum pH and optimum temperature for growth are 7.5 and 38 degrees C, respectively. The dechlorination pathway is: PCP-->2,3,4,5-tetrachlorophenol -->3,4,5-trichlorophenol-->3,5-dichlorophenol-->3-chlorophenol. This bacterium dechlorinates several different chlorophenols at ortho, meta, and para positions; exceptions to this are 2,3-dichlorophenol, 2,5-dichlorophenol, 3,4-dichlorophenol, and the monochlorophenols. The time course of PCP dechlorination suggests that two enzyme systems are involved in dehalogenation in strain PCP-1T. One system is inducible for ortho dechlorination, and the second system is inducible for meta and para dechlorinations. A 16S rRNA analysis revealed that strain PCP-1T exhibits 95% homology with Desulfitobacterium dehalogenans JW/IU-DC1, an anaerobic bacterium which can dehalogenate chlorophenols only in ortho positions. These results suggest that strain PCP-1T is a member of a new species and belongs to the recently proposed genus Desulfitobacterium. Strain PCP-1T differs from D. dehalogenans JW/IU-DC1 by its broader range of chlorophenol dechlorination. Strain PCP-1 is the type strain of the new species, Desulfitobacterium frappieri.