Although many members of the Callitrichidae, a monophyletic family of small, New World monkeys, have been observed to feed on plant exudates, available field data support the generalization that pygmy and common marmosets (Cebuella p y g m e a and Callithrix jacchus) feed on gums to a greater extent than most other callitrichids. Because microbial fermentation is required for vertebrates to digest gums, gum-feeding primates may react differently to dietary gum from their relatives that eat little gum. To test this hypothesis, digestion trials were conducted on animals from the two marmoset species, two tamarin species (Saguinus fuscicollis and S . oedipus), and a species of lion tamarin (Leontopithecus rosalia). These species span the range of body sizes within the Callitrichidae. All animals were fed two variations of a homogeneous diet, which differed only in that gum arabic was added to one. Transit time of digesta (TFA) and digestive efficiency (as measured by the coefficients of apparent digestibility of dry matter and energy [ADDM and ADE, respectivelyl) were compared between diets for each individual. As predicted, the digestive responses of marmosets differed from the responses of the other study species. In marmosets, TFA tended to be longer when gum was added to the diet, while TFA did not change in the other three species. Digestive efficiency decreased in tamarins and lion tamarins with the addition of gum to the diet; marmoset digestive efficiency was unaffected by diet. The results of this research are consistent with the hypothesis that marmosets have digestive adaptations that aid in the digestion of gum that other callitrichids lack.