Polychlorinated biphenyl (PCB) concentrations in clams (Mercenaria mercenaria) and oysters (Crassostrea virginica) from 17 stations of the western and New Bedford Harbor areas of Buzzards Bay, Massachusetts, clearly show that the New Bedford Harbor area is severely polluted. Up to 5 ppm PCBs (dry weight) were found in shellfish tissue. The most likely sources of the PCBs are chronic releases from two electrical component manufacturers in New Bedford. Close proximity of the shellfish to the source of input is indicated by a high relative abundance of the di-, tri-, and tetrachlorobiphenyls. The data suggest that the New Bedford Harbor area should be considered, along with the Hudson River and Chesapeake Bay, one of the major sources of PCB inputs to the northeastern United States coastal area.