Stable isotope analysis is now a widely-accepted tool for investigating food-web relations in aquatic ecosystems.  Isotope ratios of carbon (d13C) and nitrogen (d15N) work in a complementary fashion to identify both source importance and trophic position in complex communities consuming both aquatic and terrestrial resources.  Unfortunately, in some instances, d13C has limited ability to distinguish between aquatic and terrestrial inputs, so use of another isotope may be warranted.  Recent studies show that hydrogen isotopes (dD) may provide a suitable alternative, since terrestrial inputs are typically 100 per mil more enriched than aquatic inputs.  In this study, we investigated the potential of using dD as a food-source indicator in aquatic ecosystems by raising rainbow trout fry on two isotopically-distinct diets and in two isotopically-distinct waters to determine the relationship between: (1) diet and tissue dD, and (2) water and tissue dD.  As expected, hydrogen from the diet (78%) contributed more to tissue dD than hydrogen from water (22%), but the contribution from water was significant.  Our results suggest that, as long as: (1) the proportion from water is constant, and (2) the dD of water is measured, the dD of animal tissue will reliably identify the dD of the diet.