In the Gunung Leuser National Park of Indonesia, a population of orangutans was observed. The location of 172 night-nest sites was examined regarding their position in relation to the last food tree visited during the preceding day, and regarding to their height above the ground. There were significant differences between the sex-age classes in the distribution of distances between nest-sites and the last visited food tree. The more vulnerable animals nested at greater distances, and also tended to nest at greater height. These aspects of nest-sites selection were interpreted in terms of an anti-predator strategy; the more vulnerable animals avoid nesting in fruit trees; instead they prefer more cryptic, unpredictable and inaccessible sites.