The dietary niche and morphological adaptations of a species should be highly correlated. However, conflicting selective pressures may make predictions about diet difficult without additional knowledge of a species' life history. We tested the reliability of predicting a bat's diet from its wing morphology using data for Spix's disk-winged bat (Thyroptera tricolor). The species had been predicted to fall within either the aerial hawking or gleaning foraging group. We compared the results of a theoretical (canonical discriminant function analysis of morphology) and an applied (analysis of droppings) method of diet determination. Our results place T. tricolor in the gleaning functional group with a 77% probability according to morphology. Correspondingly, a large proportion of the diverse diet consisted of nonflying prey, such as spiders, insect larvae, and other silent prey, which should be difficult to detect using echolocation. Although some flying prey were taken, it is clear that T. tricolor regularly gleans prey from surfaces, indicating that for this species, morphology is a useful indicator of diet. However, the breadth of the diet; the high proportion of jumping spiders, leafhoppers, and insect larvae; and the extremely small size of prey were unique features of the diet that could not be predicted from morphology alone. Thus, although comparative statistical methods and the analysis of wing morphology may be helpful to predict the general ecological niche, only detailed investigation of the life history may yield the detail needed for understanding the link between morphology and ecology of individual species.