There are no published studies on the diet of Mogollon voles (Microtus mogollonensis) although this species occurs throughout the Southwest in montane forestlands. Mogollon voles are believed to be herbivorous, selecting the vegetative portion of grass as their dominant food source. Herbivores frequently select more easily digested C₃ plants over C₄ plants; we thus expected Mogollon voles would feed primarily on C₃ plants. We collected hair samples from Mogollon voles captured in northern Arizona between 1967 and 2003 and plant samples from some capture sites. Then we compared stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios to investigate dietary preferences for C₃ or C₄ plants. Mean isotope ratios for C₃ plants we sampled were −26.84‰ (s = 0.17) for δ¹³C and −0.02‰ (s = 0.32) for δ¹⁵N. For C₄ plants, mean isotope ratios for δ¹³C and δ¹⁵N were −15.04 ‰ (s = 0.38) and −0.74‰ (s = 0.55), respectively. Mogollon voles were largely herbivorous based on δ¹⁵N (mean and standard error: 3.77 ± 0.17‰) and used C₃ plants more than C4 based on δ¹³C C (−24.21 ± 0.14‰). Activities that lead to changes in plant species composition or reduction in C₃ plants in montane grasslands and forests (e.g., excessive ungulate grazing) may reduce habitat quality for Mogollon voles.