Stable isotopes can be used to evaluate trophic relationships, nutrient state, and temporal and spatial variation in diet, food webs, and behaviour both within and between species. Here we describe the development and application of models to predict habitat use of a common insectivorous bat (Eptesicus fuscus) based upon δ₁₃C and δ¹⁵N signatures of skin tissue. We used a 42-specimen sample collected from three well-characterized ecogeographic regions, disparate both in photosynthetic mechanism and fertilizer use, to generate the models. Significant univariate differences between these three sites in terms of δ¹³C (F_{2, 39} = 112.92, P < 0.0001) and δ¹⁵N (F_{2, 39} = 97.06, P < 0.0001), and multivariate significance of both variables (Wilk's λ = 0.032, F_{4, 76} = 87.02, P < 0.0001), made it possible to develop three predictive models using Fisher's linear discriminant functions: 1) a model predicting if bats forage in C₃ or mixed C₃/C₄ sites, 2) a model predicting if bats forage in agricultural areas, and 3) a combined model using both variables to predict specific habitat use. We present the results of model application to an independent dataset of 329 bats sampled from 10 states that included a broad range of δ¹³C (−26.53‰ δ₁₃C −17.20‰) and δ₁₅N (6.36‰ δ₁₅N 15.60‰) signatures. We validated the use of skin tissue samples (from wing membranes) in the model by comparing the sites used for model development across five tissue types, selecting skin samples for model development due to consistently low variance within this tissue type. Our results indicate non-specific habitat-use by big brown bats.