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The Willamette River originates in the Cascade Mountains southeast of Eugene, Oregon and makes a 300 mile northward journey through the Willamette Valley, joining the Columbia River at Portland, Oregon. The river is home to many fish and wildlife species. Large multipleuse demands placed on the river, however, have contributed to a gradual degradation in water quality. Environmental contaminants and their effects on the rivers fish and wildlife are currently being studied by researchers from the Forest and Rangeland Ecosystem Science Center and the BEST Biomonitoring Environmental Status and Trends program. This research is being conducted in conjunction with the U.S. Fish .and Wildlife Service and other federal and state agencies and is designed to identify the types of contaminants present in the Willamette River, their sources, and possible adverse effects. Contaminants such as organochlorine pesticides, polychlorinated biphenyls PCBs, dioxins, furans, diphenyl ethers, and other pollutants are suspected of causing detrimental effects in several fish and wildlife species. In previous studies of the Columbia and Willamette Rivers, environmental contaminants have been identified and quantified in river otter Lontra canadensis livers, bald eagle Haliaeetus leucocephalus eggs, osprey Pandion haliaetus eggs, and fish Henny et al. 1981, Anthony et al. 1993, Henny et al. 1996, Bucket al. 1999, Buck and Sproul1999, Kannan et al. 1999, Kannan et al. 2002, Henny et al. 2003, Buck et al. 2004, Henny et al. 2004, Villeneuve et al. 2004 . The contaminants that correlated with declines in bald eagle productivity were associated with agricultural pesticides and industrial pollutants. Historically, pesticides such as DDT and its metabolites caused severe thinning of eggshells in bald eagles, doublecrested cormorants Phalacrocorax auritus, ospreys, and many other bird species which decreased productivity and ultimately reduced the size of some populations. Mink Mustela vison are among the most sensitive mammalian species to the toxic effects of dioxins, furans and PCBs dioxins, furans and PCBs Hochstein et al. 1988, and have severely declined in numbers along the lower Columbia River Henny et al. 1981. Understanding environmental contaminant origins, sources, concentrations, and possible effects on fish and wildlife is needed to protect fish and wildlife in the Willamette River aquatic ecosystem. The river otter is a top predator of most aquatic food chains that has adapted to a wide variety of aquatic habitats, from marine environments to high mountain lakes of North America Toweill and Tabor 1982. Otters exhibit differing degrees of sociality and spacing based on habitat, shelter availability, and food abundance Reid et al. 1994. Otter home ranges approximately 11 river miles are largely defined by local topography and overlap extensively within and between sexes, exhibiting varying degrees of mutual avoidance and tolerance depending on seasonal dispersion and availability of food and shelter Reid et al. 1994. However, otter do maintain territories within home ranges that are delineated by scent marking and latrine sites. Areas within territories are used almost exclusively by the defending otter, excluding otters of the same sex i.e., female otters exclude other females and family groups, while males exclude other males. Otters eat a wide variety of fish species and aquatic invertebrates such as crabs, crayfish and mussels. Thus, river otters are a good integrator of their aquatic environments and a useful surrogate species for determining both wildlife and human contaminant exposure and potential harmful effects.