The relationship between kidney structure and diet was examined in several species of bats from the tropics. The trophic level of the bats was determined by their stable nitrogen isotopic composition (delta15 N). The results indicate that the relative thickness of the renal medulla is directly related to the trophic level of the bats. Additionally, we also determined the relationship between body mass and kidney structure. Relative medullary thickness was reciprocally related to body mass in animalivorous bats but no relationship was found in phytophagous bats. Stable isotope analysis provided a quantitative approximation of diet to test its relationship with kidney structure.