U.S. Geological Survey U.S. Geological Survey Octachlorodibenzo-p-dioxin (OCDD) was found in every bed sediment sample collected from 22 sites located throughout the Willamette Basin. OCDD concentrations in bed sediment spanned nearly three orders of magnitude, ranging from 50 ng/kg to 32,000 ng/kg. Despite this wide concentration range, a pattern emerged in the concentrations of the various congener classes: OCDD had the highest concentrations and the tetrachlorodibenzo-p-dioxin (TCDD) class had the lowest concentrations (about 100 times smaller than OCDD). Dioxin and furan concentrations in whole fish (collected from 9 of the 20 bed sediment sites) were not similar to concentrations in bed sediment from the same sites. Compared with those in the associated bed sediment, OCDD concentrations in fish tissue were much lower, and TCDD and tetrachlorodibenzofuran (TCDF) concentrations were frequently higher. Similarly, TCDD and TCDF in whole fish accounted for a larger fraction of the total dioxin-plus-furan. In addition, the predominant isomers in fish tissue had chlorine atoms in the 2,3,7, and 8 positions; other isomers were predominant in bed sediment. The differences in concentrations found in bed sediment and fish tissue cannot be explained by simple partitioning into organic carbon or lipid. Tetrachlorinated congeners and 2,3,7,8-isomers are the most toxic dioxins and furans and appear to preferentially associate with fish tissue. This work was conducted as part of the National Water-Quality Assessment Program of the U.S. Geological Survey.