The breakdown of polychlorinated biphenyl (PCB) congeners in situ in sediments heavily contaminated with PCBs by processes called reductive dechlorinations have been reported. These studies characterized several distinct dechlorination patterns, caused by different strains of anaerobic bacteria, which resulted in PCB residues that were altered from the original Aroclor inputs. The upper New Bedford Harbor (NBH), above the Coggeshall St. Bridge, is a shallow, approximately 200-acre salt marsh estuary, which received large inputs of Aroclor 1254 (A-1254) and Aroclor 1242 (A-1242) from 1947 to 1970, and possibly Aroclor 1016 (A-1016) from 1970 to 1978, from a capacitor manufacturing plant designated plant A. Another study found variations in the extent of dechlorination processes in 5- to 7.5-cm and 15- to 17.5-cm sections of cores taken in the northern part of the upper NBH. However, the distributions of PCBs in extracts of sediment core sections taken in the southern part of the upper NBH as part of a pilot dredging study at the Environmental Research Laboratory-Narragansett (ERLN) showed only small alterations relative to mixtures of A-1242 and A-1254. The present study was undertaken to determine the extent of alteration of PCB residues in the sediments of upper NBH resulting from dechlorination processes, and to estimate the rates of these processes.