At the population-level, sea otters showed substantial variation in both δ13C and δ15N values, occupying nearly all of the “isotopic space” created by the diversity of isotopic signatures of potential prey taxa. Most of the variation in sea otter vibrissae was accounted for by differences between individuals, with much less contributed by within-individual variation. A majority of sea otters (~80%) showed relatively little temporal variability in isotopic composition, suggesting that the proportional composition of most individuals’ diets is relatively constant over time; a few individuals (~20%) exhibited a high degree of intra-vibrissae isotopic variability, suggesting seasonal shifts in diet composition. These results and our interpretation of them were supported by long-term observational data on the diets of radio-tagged sea otters from the same population (n=23). Our results demonstrate that stable isotopes can provide an efficient tool for measuring individual- and population-level dietary breadth, and may be useful for studying populations where longitudinal data on individuals would otherwise be impossible to acquire. This will be critical for examining the causes and consequences of dietary variation within and among consumer populations, thereby improving our understanding of these important ecological and evolutionary processes at the community level.