We evaluate the hardness of foods consumed by sympatric Cercopithecus ascanius (redtail guenons) and Lophocebus albigena (grey-cheeked mangabeys), and consider how selection might operate to influence foraging adaptations. Since L. albigena has among the thickest dental enamel in extant primates and is commonly referred to as a hard-object consumer, we predicted that their diet would be harder than that of the guenon. Data on diet and food hardness (as measured by resistance to puncture and crushing) were collected between June–October of 1997 at Kibale National Park, Uganda, and were compared to similar data collected in Kibale between 1991–1994. Contrary to what we predicted, there was no difference in dietary hardness when the puncture resistance of all fruit consumed by the two species was compared (31 tree species in both study periods). However, in June–October 1997, L. albigena exploited a diet more resistant to puncture and crushing than C. ascanius. This difference is largely explained by the higher percentage of bark and seeds consumed by the L. albigena during this period. We suggest that it is the difference in the mechanical properties of fallback foods during critical periods that may have served as the selective pressure for thick enamel in L. albigena. Am J Phys Anthropol, 2004. © 2004 Wiley-Liss, Inc.