Members of the Cercocebus-Mandrillus clade are united by several morphological features, including expanded premolars which are argued to be associated with a preponderance of hard objects in the diet. We test the association between premolar expansion and hard object feeding by examining how different dental regions are used during food processing. We examined the diet and oral processing activities of sooty mangabeys (Cercocebus atys) in the Ivory Coast's Tai forest from August 2008 to September 2009. In addition to compiling diet profiles, we recorded the frequency that individuals performed four activities: 1) incising, 2) canine puncturing/scraping, 3) postcanine crushing (i.e., isometric biting), and 4) routine mastication (chewing cycles). Sooty mangabeys have a relatively narrow diet that consists largely of nuts/seeds, fruits, and invertebrates. While there are age and sex differences in diet, the most frequently consumed foods are similar across age and sex classes. The most frequently consumed foods are seeds of Sacoglottis gabonensis which are the hardest items in the sooty mangabey diet. Patterns of ingestive behavior vary with food type, but adults and nonadults (excluding infants dependent onmothers) of both sexes process similar foods. Premolar expansion in Cercocebus atys is associated with powerful crushing of hard objects of specific size and durophagy is a constant feature of sooty mangabey feeding ecology throughout ontogeny. Am J Phys Anthropol, 2010. © 2010 Wiley-Liss, Inc.