Describes a study of the stable isotope composition of sulphur, carbon, and nitrogen in the tissues of fish from two locations in the Athabasca River and two on the Peace River. Fish species analysed included burbot, walleye, mountain whitefish, northern pike, goldeye, longnose sucker, and lake chub. A set of samples consisting of biofilm, invertebrates, and fish from the upper Athabasca was also analysed. The purpose of the study was to extend the data base on feeding and movement of fish which could be derived from the carbon and sulphur isotope data, and to use the nitrogen isotope data to define the trophic positions of the organisms. Isotope analyses of water samples established the isotope signals of the source of organic matter produced in or carried into the Athabasca by its tributaries so that the dependence of the food chain on those sources could be assessed