The taxon known as Cercopithecoides williamsi has been reconstructed as a terrestrial primate folivore from its dental morphology and heavy dental microwear. It was recovered at a number of Plio-Pleistocene caves in South Africa, including Makapansgat, Sterkfontein Member 4, Sterkfontein West Pit and Bolt’s Farm. For the purposes of corroborating or contradicting previous dietary reconstructions, six specimens of Cercopithecoides williamsi were examined and compared with the extant primate taxa Papio ursinus (n = 5), Papio cynocephalus (n = 5) and Colobus polykomos (n = 5) to provide a frame of reference, using low-magnification stereomicroscopy in conjunction with an ocular reticle (0.04 mm x 0.04 mm) as a standard sampling strategy. Least Square Means derived from Analyses of Variance and Tukey’s post-hoc tests of significance detected significant signals in the dataset. Papio ursinus is significantly distinct from the other specimens in its low pit count and high scratch frequency, indicative of the consumption of grass blades and underground storage organs. Colobus polykomos is significantly distinct in its high number of pits and low scratch frequency, indicating folivory. Cercopithecoides williamsi lies between these two extremes by exhibiting moderate amounts of pits and relatively low numbers of scratches, which is more similar to Colobus polykomos, corroborating prior inferences of folivory in this taxon. The Sterkfontein West Pit specimen, SWP 495, however, is somewhat variant from the other Cercopithecoides williamsi individuals in its heavy scratch count, indicating a mixed diet, perhaps from a drier habitat at the terminal Pliocene or early Pleistocene.