Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Being active in the same environment at different times exposes animals to the effects of very different environmental factors, both biotic and abiotic. In the present study, we used live traps equipped with timing devices to evaluate the potential role of biotic factors (competition and food abundance) on overall overlap in the temporal niche axis of 4 insectivorous small mammals in high-elevation grassland fields (campos de altitude) of southern Brazil. Based on resources availability (invertebrates), data on animal captures were pooled in 2 seasons: scarcity (June 2001September 2001) and abundance (November 2001May 2002) seasons. We tested for non-random structure in temporal niche overlap among the species in each season. These species were the rodents Oxymycterus nasutus (Waterhouse, 1837), Deltamys sp., Akodon azarae (Fischer, 1829), and the marsupial Monodelphis brevicaudis Olfers, 1818. The studied community was mainly diurnal with crepuscular peaks. Simulations using the Pianka index of niche overlap indicated that the empirical assemblage-wide overlap was not significantly different from randomly generated patterns in the abundance season but significantly greater than expected by chance alone in the scarcity season. All the species showed an increase in temporal niche breadth during the abundance season, which appears to be related to longer daylength and high nocturnal temperatures. Patterns on both temporal niche overlap and temporal niche breadth were the opposite to those that we were expecting in the case of diel activity patterns determined by competition for dietary resources. Therefore, we conclude that competition did not seem to be preponderant for determining patterns of temporal niche overlap by the studied community.64375386British Ecological Society [1841]UNISINOSFAPERGS [0112763]Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)British Ecological Society [1841]FAPERGS [0112763]CNPq [300286/99-6]