Polychlorinated Biphenyls (PCB's) are organochlorine molecules which find various industrial and product applications. PCB's are of concern to biologists because they are toxic substances which have become global contaminants. They are also of concern to biologists and analytical chemists because they interfere with the determination of some organochlorine pesticide residues. PCB's were discovered to be environmental contaminants after they showed up as unidentified peaks in pesticide analysis using gas-liquid chromatography with an electron capture detector (GLC-EC). In the present experiment standard GLC-EC techniques were used to assay PCB's in Daphnia experimentally contaminated in the laboratory and in crayfish from the Willamette River. Daphnia were placed in water containing 0.1, 4, 50 or 100 parts per billion (ppb) PCB for 4 to 72 hours. There was no mortality in any of the experiments. The individual PCB compounds were apparently taken up equally, since relative peak heights were similar in the standard and the residues extracted from Daphnia. Final concentrations of PCB's in Daphnis ranged from 1200 times that of the water (at water concentrations of 100 ppb) to 104,000 times that of water (at water concentrations of 0.1 ppb). The lower concentrations used here approximate environmental PCB levels found in some areas of the U.S. and elsewhere. Assuming that chronic exposure to these lower concentrations would not strongly inhibit growth or reproduction of Daphnia,, the high biological magnification found here would suggest that Daphnia and related organisms may play an important role in the accumulation of PCB's in fresh water food webs. Digestive glands of crayfish taken from the Willamette River were analyzed for PCB's. Statistically significant regional differences were found in the concentrations of PCB's in these organisms in the lower Willamette Valley. Crayfish from the center of Portland have the highest residues (7 to 9 parts per million). A similar distribution has been previously reported for other urban areas. In the present investigation, however, the highest residues occurred in crayfish from the river at a point approximately in the center of the city itself, but two miles upstream from the major industrial areas. Thus, in this case, the major source of environmental contamination may have been released from manufactured goods (e.g., automobile tires, paints, etc.) rather than from industrial sewerage. I