Species of carnivorans that eat bone are believed to break teeth more often than those eating mainly meat. Two predictions that arise from this hypothesis are that bats, which do not eat bone, will have a lower incidence of broken teeth than carnivorans and that smaller carnivorans, which often feed heavily on arthropods and other insects, will exhibit less tooth damage than larger carnivorans. We found no difference between bats and carnivorans in the proportion of skulls with broken or missing teeth in a sample including 10 species of carnivorans from ermines to bears, and 13 species of bats including some that eat insects, fruit, nectar and pollen, and blood. When the sample is expanded to include larger carnivorans studied by Van Valkenburgh (1988), there is a significant difference in the incidence of tooth breakage, indicating that the selection of species in the sample affects the results. Tooth breakage was strongly influenced by body size in carnivorans but not in bats. This evidence suggests that factors other than diet influence tooth breakage in mammals. We propose that lifespan, which increases with body size in carnivorans but not in bats, is a better predictor of tooth breakage than diet.