Modern datasets provide the context necessary for accurate interpretations of isotopic data from archaeological faunal assemblages. In this study, we use the oxygen isotope ratios (δ¹⁸O) of modern small mammals from Chaco Canyon, New Mexico, to quantify expected isotopic variation in a local population. The δ¹⁸O values of local, modern small mammals encompass a broad range (−6.0‰ to 4.8‰ VPDB), which is expected given the extreme seasonal variation in the δ¹⁸O of precipitation on the Colorado Plateau (−11‰ to −3‰ VPDB). Isotopic ratios of small mammals obtained from excavated archaeological sites in Chaco Canyon (ca. AD 800 to 1200) show no significant differences with their modern counterparts, suggesting that there is no difference in the origins of the archaeological small-mammal collection and the modern, local Chaco Canyon small-mammal collection. In contrast, δ¹⁸O values of large mammals from Chaco archaeological sites are significantly different from those of modern specimens, reflecting a nonlocal, but also nonspecific, source in the past.