Common marmosets (Callithrix jacchus penicillata) living in a central Brazilian gallery forest depend heavily on exudates as a food. The marmosets gnaw holes in the bark of certain tree species, and all exudate collection takes place at these holes. Marmosets thus forage as ectoparasites on these trees. In all, 14 species of trees were parasitized by the marmosets; however, almost all activity was concentrated on three species: Vochysia pyramidalis, Callisthene major, and Tapirira guianensis. Marmosets foraged more on larger trees near the center of the gallery forest. Although Callisthene was utilized in a somewhat different fashion than the other two species, there was no strong difference in preference among the three important species. The exudate sources were distributed regularly among the gallery forest.