THE nitrogen pools of animals are enriched in N-15 relative to their food(1), with the top predators having the highest concentrations of this stable isotope(2). The use of delta(15) N to indicate trophic position depends on the degree to which it reflects variation in the underlying food-web structure, rather than variable fractionation along the food chain. Here we compare adult lake trout, a top pelagic predator, from a series of lakes, and find that delta(15)N values vary from 7.5 to 17.5 parts per thousand, a surprisingly wide range for one species. The length of the food chain can explain this variation, supporting the idea that delta(15)N is a food-web descriptor. Food-chain length was measured by the presence or absence of two intermediate trophic levels, pelagic forage fish and the macrozooplankter, Mysis relicta, each of which when present contributes about three delta(15)N units to the trout signature. We find that delta(15)N can be used as a continuous, integrative measure of trophic position, which is supported by its correlation to mercury levels in lake trout.