Ranging and foraging patterns observed during a field study of the behavioral ecology of Ateles belzebuth belzebuth are analysed in the context of temporal and spatial variation in the distribution of plant resources at the study site. The study group exhibited two basic patterns of foraging behaviour and habitat exploitation, which appeared to correlate with the observed variation in resource availability. During the dry season, when the relative abundance of fruit decreased, the spider monkeys ranged over a smaller area, repeatedly visiting a few intensively used fruiting and sleeping trees. As fruit abundance increased during the course of the wet season, the study group expanded its range, and both the daily path length and the number of fruiting trees visited increased daily. These patterns are analysed and evaluated in relation to foraging strategies.