Mirex is a persistent chlorinated hydrocarbon found in appreciable concentrations in sediments and pelagic organisms of Lake Ontario. Concentrations are particularly high in introduced salmonids, which spawn in considerable numbers in tributaries of the Lake. We sampled the sediments and biota of several tributary streams in the Salmon River drainage basin. Greater concentrations of mirex were found in the resident brown trout (Salmo trutta) of tributaries accessible to migrating salmonids than in a nearby reference tributary inaccessible to migration. No significant differences in mirex concentrations were found in lower trophic levels (crayfish and stoneflies) or sediments, although mirex was detected in all samples. Because point sources of mirex are distant from these tributaries we conclude that migrating salmonids make a significant contribution to the upstream transport of mirex from Lake Ontario. Ingestion of salmonid eggs by brown trout, decomposition of salmonid carcasses by blowfly larvae, and ingestion of carcasses by aquatic and terrestrial scavengers are all means by which the contaminant is introduced to upstream environments.