The diet of maned sloths was studied throughout 14 months in an Atlantic forest reserve of south-eastern Brazil. Three adult sloths were observed for a total of 680 h and located monthly by radio-telemetry. Data were collected on diet recording the actual time the sloths spent eating plant species. Overall the diet was composed of 99% leaves with young leaves (68%) preferred to mature ones (7%) throughout the year. A higher proportion of tree leaves (83%) than liana leaves (16%) were included in the diet. When analysed together the diet of the three animals included a total of 21 plant species (16 tree and 5 liana) but each individual made up its diet with an even smaller number of species (7–12) and with a particular subset of the local flora. This is a very small portion of the total number of tree and liana species available to the sloths; furthermore the top species consumed were present at very low population densities in the forest. Thus B. torquatus like other congeneric species studied elsewhere in the Neotropics is a strictly arboreal folivore with a highly selective diet probably resulting from evolving physiological adaptations to cope with a smaller range of plant secondary compounds. This is possible for the species of this genus through a combination of low basal rates of metabolism which enable the sloths to survive on an energy-poor diet and a very long passage time of digesta which in turn aids the digestion of a fibre-rich diet while possibly contributing to the degradation of secondary compounds.