The time required for the microbially mediated dechlorination of chloroaromatics in anaerobic freshwater sediments may be dependent upon sediment microbial communities and physical characteristics, as well as chlorine position in the aromatic ring. To better understand such possible relationships, the authors studied the reductive dechlorination of three dichlorophenol isomers in anaerobic sediments collected every other month for a year from five sites in one pond. Numbers of specific dechlorinating microorganisms were estimated by a most probable number technique, and sediment temperature, pH, E(sub h), and organic carbon content were determined. Levels of dissolved organic carbon, sulfate, and nitrate also were quantified in water samples collected with the anaerobic sediments. The relative susceptibility of the 2,4-, 2,5-, and 3,4- isomers to dechlorination was determined in sediment slurries. Dechlorination to monochlorophenols required from 2 to more than 32 weeks for completion, with the relative rates being 2,4->2,5->3,4-. The number of specific dechlorinating microbes appears to be important in determining the time required for this transformation.