Oncorhynchus nerka occur both as anadromous sockeye salmon that spend most of their life in the ocean, and as non-anadromous kokanee salmon that remain in fresh water their entire lives. We assessed whether stable isotopes of sulfur (δ 34 S) in otoliths could be used to distinguish sockeye salmon and kokanee ecotypes that are otherwise difficult to identify when they share a common freshwater rearing environment. We also investigated the chemical link between salmon and their diet by measuring δ 34 S in various fish tissues (eggs, muscle, scales) and zooplankton. δ 34 S (mean±SE) in sockeye salmon eggs (18.7±0.4‰) and marine zooplankton (20.5±0.1‰) were enriched by 10-14‰ compared with kokanee eggs and freshwater zooplankton. δ 34 S in the otolith cores of sockeye salmon (19.2±0.7‰) and kokanee salmon (5.3±1.1‰) were similar to δ 34 S in marine and freshwater zooplankton, respectively, indicating that the core is derived from maternal yolk tissue and reflects the maternal diet. δ 34 S in the freshwater growth zone of otoliths did not differ significantly between sockeye (5.9±1.1‰) and kokanee salmon (4.4± 1.2‰), and was similar to freshwater zooplankton. The mean difference between δ 34 S in the otolith core and first year of growth was 13.3±1.4‰ for sockeye and 0.65±1.3‰ for kokanee salmon. A quadratic discriminant function developed from measurements of δ 34 S in otoliths of known maternal origin provided perfect classification rates in cross-validation tests. Thus, sulfur isotope ratios in otoliths are effective in discriminating between anadromous and nonanadromous ecotypes of O. nerka.