Incorporation of stable isotope analysis (SIA) into routine environmental effects monitoring programs of receiving waters may enable determination of the spatial extent of biotic exposure and discrimination among sources of complex effluents. To evaluate this hypothesis, longnose sucker (Catostomus catostomus) were collected from four sites along the Athabasca River, Alberta (upstream reference site, two sites downstream of effluents from two pulp and paper mills, and a site downstream of effluent from a municipal sewage treatment plant). Stable isotopes of carbon, nitrogen, sulfur and chlorine were analyzed in bone, gonad, liver and white muscle tissues of the fish. In general, most sites and tissues differed according to δ13C, δ15N and δ34S values. Also, an interaction between site and tissue was observed for δ15N values. A better insight into the usefulness of stable isotopes was obtained through the use of multivariate discriminant function analysis. δ15N and δ34S signatures of gonad and liver tissues of males were most effective at classifying fish according to site (~70% for both tissues). For all tissues except bone, fish from the upstream reference site were most separable from all others, especially females. δ37Cl values for female gonads and male livers were related to sites downstream of the pulp and paper mills. Future research should routinely include SIA of fish tissues, but also of effluents, receiving waters and food web components to better resolve links between specific effluents and fish exposure.