Among fossil primates, thick enamel has been interpreted as an adaptation to hard-object feeding. However, correlations between thickness of enamel and diet have not been investigated rigorously among extant taxa. Thickness of enamel was compared within and between congeneric pairs that feed on hard and soft objects from two primate and three chiropteran families. Within each pair, the hard-object feeder exhibits relatively thicker enamel than its congener that feeds on softer items. However, there is overlap in both types of feeders, and the hypothesis that a specific value of enamel thickness be used to separate hard- and soft-object feeders is rejected. Dietary inferences based on thickness of enamel should be made only within an appropriate taxonomic context.