Bat researchers have used both morphological indices and total body electric conductivity (TOBEC) as proxies for body condition in a variety of studies, but have typically not validated these indices against direct measurement of body composition. We quantified body composition (total carcass lipids) to determine if morphological indices were useful predictors of body condition in big brown bats (Eptesicus fuscus). We also evaluated body composition indirectly by TOBEC using EM-SCAN^® technology. The most important predictors of body composition in multiple regression analysis were body mass-to-forearm ratio (partial r² = 0.82, P < 0.001) followed by TOBEC measurement (partial r² = 0.08, P < 0.001) and to a minor extent head length (partial r² = 0.02, P < 0.05). Morphological condition indices alone may be adequate for some studies because of lower cost and effort. Marking bats with passive integrated transponder (PIT) tags affected TOBEC measurements.