Polychlorinated biphenyls (PCBs) are important environmental contaml- nants. They are widely distributed and have been associated with toxic effects in wildlife, such as deformities and lowered reproductive success (1 ), and with hepatic tumors in rats (2 ,3 ). They can enter the human food supply through fish, which bloaccumulate PCBs (4 ). A large fraction of the environmental PCBs are in sediments (5 ). It was long thought that PCBs in sediments remain unaltered, but research over the last few years has demonstrated that this is not always true. Under suitable conditions, lesser chlorinated PCBs in surficial sediments may be aerobically degraded (6 ,7 ), and PCBs deeper in sediments may be reductively dechlorinated by anaerobic bacteria (8 -19 ).