Mycophagy is a relatively rare behavior in primates and has only been recorded in five callitrichid species. Here, we present data on the feeding ecology of a free-ranging group of Callithrix flaviceps, which was studied in the Augusto Ruschi Biological Reserve, Southeastern Brazil, in 2008. In contrast with other marmosets, which are typically gummivorous, the study group was predominantly mycophagous–insectivorous, with fungi corresponding to 64.8% of total feeding records, and gum (6.1%) and fruit (3.3%) together providing only a minor part of the diet. Prey corresponded to 25.8% of the group's diet. The fungi (Mycocitrus spp.) consumed by the marmosets were found attached to the stems of Merostachys bamboo. As the animal component of the group's diet was similar to that recorded in studies of other marmosets, we propose that fungi were exploited primarily as a substitute for plant material, in particular exudates. This highly mycophagous diet may be determined by two principal factors: (1) the abundance of fungi within the study area, and (2) the avoidance of bark gouging, for which C. flaviceps may be less specialized than most other marmosets. These conclusions are supported by comparisons with other marmoset groups, which indicate an ecological specialization for mycophagy in C. flaviceps, and that the species will resort to gummivory in habitats where fungi are scarce. Am. J. Primatol. 72:515–521, 2010. © 2010 Wiley-Liss, Inc.