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The semi-arid Caatinga of the Brazilian northeast is a fragile, poorly known scrubland biome, which suffers increasing anthropogenic impacts. While 143 mammal species are known from the biome, few data are available on community composition or population parameters. In the present study, a 115-ha fragment of arboreal caatinga was surveyed in northern Sergipe in 2009 using standard line transect procedures. Total transect length was 133 km, and density estimates were calculated using sighting functions. Additional information on the occurrence of mammalian species was collected non-systematically. Nine species, including three carnivores, were confirmed in the study area, but only three were recorded in the surveys, two of which [the critically endangered blond titi, Callicebus barbarabrownae (Hershkovitz 1990), and the rock cavy, Kerodon rupestris (Wied-Neuwied 1820)] are endemic to the Caatinga. The third species was the common marmoset, Callithrix jacchus (Linnaeus 1758). C. jacchus and K. rupestris were relatively abundant, with estimated densities of 169.7 and 116.7 individuals per km(2), respectively, but C. barbarabrownae was rare, and the local population was estimated to contain only five individuals, i.e., approximately four individuals per km(2). If typical, densities this low would probably threaten the viability of C. barbarabrownae populations at most, if not all sites in which the species still occurs.