Between May, 1975 and April, 1977 eleven agricultural watersheds ranging in size between 19 and 79 km 2 were studied for residues of persistent organochlorine insecticides and PCB. Stream water, rainwater, sediments, invertebrates and fish were analysed. The presence of DDT, its metabolites and dieldrin, insecticides that had been removed from field application between 1969 and 1972, and PCB, an industrial fire retardant that had been voluntarily restricted to ‘closed system’ use in 1971, were identified. ΣDDT was present in 93% of 949 initial stream waters and 40.6% exceeded a 3 ng/l International Joint Commission (IJC-1977) water quality objective for boundary waters of the Great Lakes. Unit area loading for ΣDDT ranged from 2.9 to 88 mg/ha/yr per watershed. Dieldrin was present in one-fifth of stream waters and 16% exceeded the 1 ng/l IJC (1977) objective. Unit area loading for dieldrin varied from zero to 28 mg/ha/yr per watershed. PCB residues were detected in 94.4% of stream waters at or above 2 ng/l and unit area loading varied from 28 to 269 mg/ha/yr per watershed. DDE and PCB were found in rainwater and this could represent a major source of contamination of PCB. The soil reservoir was the main source of DDT and dieldrin. Accumulations of all three organochlorine compounds in invertebrates and fish were between 10 3 and 10 4 times. None of the fish or invertebrates exceeded the IJC objective of 0.1 μg/g PCB.