ABSTRACT. Theoretical considerations suggest that the ability to detect the presence or approach of a predator when there is still enough time to flee (early detection) should improve with group size, if group living is to be advantageous for individual non-human primates. The hypothesis that the distance at which forest primates detect predators increases with the size of their party was confirmed by observation. It was found that in addition to party size height (vegetation density) could also influence detection distance. Because height relates not only to visibility but also to the number of potential predators, one would predict that small parties are found higher in the canopy to compensate for the increased risk of predation. This prediction was confirmed using data on long-tailed macaques (Macaca fascicularis).The correlation found between party size and predation risk demonstrates that forest monkeys can adjust their behaviour in response to changes in predation risk, and hence support the hypothesis that predation risk has been an important, perhaps even the only, selective force responsible for the evolution of group living in non-human primates.