During a three-year field study of the socio-ecology of Sumatran orang-utans, their use of the canopy was investigated in the Gunung Leuser National Park, Indonesia. This paper concerns the positional behaviour of different age-sex classes of orang-utans. Adolescents and females with infants differed significantly from an adult male in the following respects: the use of locomotion types (more 'quadrumanous scrambling' and perhaps also 'quadrupedal walking' and less 'tree swaying'); substrate use during resting, and travelling and resting heights. We suggest that large body size restricts the travel route options in higher forest strata and necessitates the use of the lower stratum. Here, 'tree swaying' is an efficient method of progression, particularly for heavy animals. Mothers with infants are forced to travel in the lower zones as well. The fact that they return to a greater heights when they go to rest might suggest that they travel lower in spite of a greater predation risk.