Stable isotope ratio analysis (SIRA) of carbon (δ13C) and nitrogen (δ15N) in tissue samples of herbivores can identify photosynthetic pathways (C3 vs. C4) of plants consumed. We present results from free-ranging Rocky Mountain elk (Cervus elaphus) that highlight the ability to differentiate diets using tissue δ13C and δ15N. The signatures of δ13C and δ15N differed in tissues of varying metabolic activity: muscle, a short-term dietary indicator (i.e., 1–2 months) and hoof, a long-term dietary indicator (i.e., 3–12 months). We also documented that δ13C and δ15N values along elk hooves (proximal, middle, distal sections) elucidated temporal shifts in dietary selection. The carbon isotopes of the composite hoof were similar to those of the middle section, but the composite hoof differed in δ13C from the distal and proximal sections. The δ13C and δ15N signatures also differed among elk populations, indicating temporal dietary shifts of individuals occupying disparate native range and human-derived agricultural landscapes. Analyses of stable isotopes in various tissues highlighted carbon and nitrogen assimilation through time and differences in the foraging ecology of a rangeland herbivore. Published in 2009 by John Wiley & Sons, Ltd.