Species of fish [18] from Lake Saint Clair and 19 species from Lake Erie were analyzed for organochlorine pesticides and polychlorinated biphenyls (PCB) [environmental pollutants] between 1968 and 1976. Mean residues of .SIGMA.DDT peaked at 1.19 ppm in longnose gar (Lepisosteus osseus) caught in Lake Saint Clair in 1970-71, but declined in all species by 1975-76. Dieldrin levels in fish tissues increased over the same period. White bass (Morone chrysops), caught in 1975 in Lake Erie, had the highest mean residue of dieldrin at 0.17 ppm. PCB residues increased in some species and decreased in others. PCB residues exceeding the tolerance level of Health and Welfare Canada were found in the following: from Lake Saint Clair, smallmouth bass (Micropterus dolomieui) in 1975 and channel catfish (Ictalurus punctatus) in 1971; from Lake Erie, coho salmon (Oncorhynchus kisutch) in 1970, smallmouth bass, alewife (Alosa pseudoharengus), freshwater drum (Aplodinotus grunniens) and gizzard shad (Dorosoma cepedianum) in 1971 and white bass in 1971 and 1976. Sediments in Lake Erie were 5-10 .times. more highly contaminated with .SIGMA.DDT, dieldrin and PCB than sediments from Lake Saint Clair. .SIGMA.DDT and dieldrin residues in fish tissues did not necessarily reflect this trend, but PCB were higher in fish from Lake Erie.