Forelimbs in many mammals are used in both locomotion and the acquisition of food. In bats, the forelimbs are highly specialized for flight, but the thumb is free to preform other functions. Using morphological data from museum specimens and field observations of feeding behavior, I documented how Neotropical leaf-nosed bats (family: Phyllostomidae) use their thumbs during feeding, and examined how thumb structure and function are linked with diet. I then used these data to test when changes in thumb morphology and feeding behaviour evolved, focusing on two specific time points: the evolution of frugivory in phyllostomids; and the diversification point at the base of the clade Stenodermatinae, which are hard canopy fruit specialists. I found that size-corrected thumb length was weakly associated with diet, but the frequency and duration of thumb use were higher in frugivorous species than in their insectivorous counterparts. Likewise, the function for which frugivores used their thumbs differed from insectivores, although variation in specific thumb movements could not be teased apart in the analyses. When I modeled selective regimes of diet, I found that species that consumed fruit showed selection for increased frequency and duration of thumb movements, but there was no evidence of selection for increased thumb length at either of the two evolutionary time points tested. This suggests that a shift in how thumbs are used during feeding, and not thumb morphology, may be linked to a transition from a mainly insectivorous diet to frugivory in phyllostomid bats.