Technical Abstract: Stable isotope analysis of diet switching by fishes is often hampered by slow turnover rates of the tissues analyzed (usually muscle or fins). We examined epidermal mucus as a potentially faster turnover “tissue” that might provide a more rapid assessment of diet switching. In a controlled hatchery experiment, we switched the diet of juvenile steelhead (sea-run rainbow trout) from a plant-based feed with a low 13C and 15N to a fish-meal based diet with higher delta values. We found mucus to provide a significantly more rapid response to diet switching than muscle tissue, even for growing juvenile fish. Mucus may provide a rapid turnover tissue for analysis of diet (or habitat) switching by fish. It has the additional advantage that it may be sampled non-lethally in some fishes, thereby avoiding problems in studying threatened and endangered species. This is the first report of the use of mucus in stable isotope analyses of fish tissues.