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Fish with no detectable levels of the contaminant mirex were grown in Lake Ontario waters under conditions simulating commercial aquaculture. Benthic black bullheads (Ameiurus me/as) were grown in cages placed in a bay of the lake. Pelagic rainbow trout (Oncorhynchus mykiss) were grown in terrestrial raceways served with Lake Ontario waters. Contaminant-free fingerlings were reared to a large size on a commercial ration in these systems, which partially isolated them from the contaminant-laden food web and bottom sediments. Black bullheads fed a mirex-spiked, commercially prepared food had mirex concentrations that exceeded the U.S. Food and Drug Administration (FDA) action level of 0.1 p,g/g, significantly higher than concen, trations in fish receiving the same commercial food without mirex. Ninety percent offish receiving the unspiked ration had nondetectable levels of mirex (values below 0.002 p,g/g). The 10% containing mirex had concentrations 94% below FDA action level. In the rainbow trout study, 97% of the fish had no detectable levels ofmirex. This investigation demonstrated that bioaccumulation of the lipophilic contaminant mirex by fish cultured under simulated commercial conditions in Lake Ontario waters was not significant. These findings have implications for commercial aquaculture, regulatory decisions, and health-conscious fish consumers in the Great Lakes Basin.