Ecologically similar and closely related species in sympatry may differ in their activity patterns to avoid interspecific contest competition. We here present the 1st study aimed at evaluating the effects of seasonality, group size, and presence of a congener on the activity patterns of 2 syntopic species of howler monkeys, the brown howler (Alouatta guariba clamitans) and the black-and-gold howler (A. caraya), in northeastern Argentina. During 12 months, we collected activity data on 2 groups of each species characterized by different sizes and degrees of home-range overlap, together with data on food availability. We analyzed seasonal variation in activity budgets and daily activity patterns, as well as the relationships between monthly activity budget and food availability, diet, and climate, both within and between species. Black-and-gold howlers, more clearly than brown howlers, adopted an energy-minimization strategy, reducing costly activities such as moving and traveling, during the lean season. Within each howler species, individuals in the largest group showed a greater proportion of time spent moving or traveling, or both, compared to individuals in the smallest group, suggesting the existence of within-group food competition. Juveniles of both species rested less and moved and socialized more than adults. Overall, black-and-gold howlers spent proportionately more time moving and traveling, and less time resting, probably due to their larger mean group size, than brown howlers. Daily feeding peak times diverged only slightly among groups in the lean season, but differences between groups of different species with overlapping home ranges were not larger than those among scarcely or nonoverlapping groups (of same or different species). In conclusion, even though the 2 howler species showed differences in activity patterns, mainly related to differences in their response to food seasonality, and in group size, we found no evidence of time partitioning.