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We studied factors affecting variation in home-range size of four groups of bare-ear marmosets (Callithrix argentata) in patches of forest within a central Amazonian savanna. We determined relative use of different parts of the home range by radiotelemetry. We estimated fruit availability monthly in transects through each home range and mapped the habitats within the home range of each group. We determined the densities of gum- and fruit-producing trees in 18 50 × 50-m quadrats and related these data to the frequency of use by marmosets. Home-range size varied by a factor of 6 between groups, even though the study area covered <15 km2 and included only one major biome. Marmoset activity was concentrated in areas with many gum-producing trees. Monthly range size is positively correlated with fruit availability only for the group with the largest home range; the other groups appeared to be responding to other factors. Home-range sizes appeared to be limited by the size of the main patch of contiguous forest available to each group. Our findings suggest that conservation planning that does not consider the possibility of large differences among primate home-range sizes may be unsatisfactory.