The sensitivity of a biomarker for environmental contamination with persistent organochlorines and PAHs (cytochrome P450-1A1 induction in feral fish) was evaluated at six sites along 314 km of the Willamette River, Oregon. Whole-body contamination of Northern squaw-fish with approximately 2 pg/g of TCDD and 20 pg/g of TCDF in Portland Harbor was significantly higher than upstream sites. Carp muscle TCDF and hepatic biomarker response were also elevated at this site, as were sediment TCDD, TCDF, and PAHs concentrations. In July, cutthroat trout TCDD concentrations above and below a bleached kraft pulp mill were similar. In October cutthroat trout below the mill contained about 2-fold more TCDD than fish upstream