We used stable isotope techniques to determine the nitrogen and carbon isotopic composition of 21 species of neotropical bats (17 phyllostomids, two mormopids, one molossid and one emballonurid) representing a diverse array of feeding habits (e.g. frugivory, nectarivory, insectivory, carnivory, and sanguinivory) as well as the isotopic composition of plants and insects presumably ingested by the bats. We found trophic enrichment of both 13C and 15N, although 15N enrichment was lower than expected between frugivorous and insectivorous bats. Our data indicate that most species of frugivorous bats examined have a mixed diet of fruits and insects and only Centurio senex, Artibeus lituratus and Dermanura watsonii appear to be exclusively frugivorous. One species of insectivorous bat, Tonatia evotis, apparently relies on fruit as part of its diet. Although preliminary, our results indicate that stable isotope methods can be used to quantify the importance of different dietary classes in the diets of bats.