Previously, wild orangutan feeding and ranging behaviors have been described only from populations in hilly or mountainous regions. The Tanjung Puting study focuses on an orangutan population in a swampy lowland area near sea level. Tanjung Puting also differs from other areas in the virtual absence of large figs, which are significant orangutan food sources elsewhere. During a 4-year period and 6804 hr of observation, focal orangutans were recorded in 11,338 foraging bouts accounting for 3805 hr. Composition and phenology of the forest habitat were documented. The orangutans were predominantly frugivorous, with fruit-eating accounting for 61% of the foraging time. However, the overall variety in their diet was remarkable; 317 different food types have been identified, including fungus, insects, and honey. Orang-utans were strongly opportunistic foragers, with the composition of their diet varying markedly from month to month. During most months orangutans fed on a complex mix of fruit, leaves, bark, insects, and small vines. During some months fruit was not the major component of the diet. All orangutans foraged in both the dry-ground mixed dipterocarp forest and the peatswamp forest habitats found in their ranges. Adult males and females utilized different proportions of certain resources in their diets. Prime adult males also ranged further per day and spent more time on the ground than prime adult females. At Tanjung Puting contact with other orangutans usually increased a focal orangutan’s day length, day range, and amount of time spent moving. This suggests that foraging alone maximized each orangutan’s foraging returns by minimizing the day range traveled. Orangutan solitariness is the result of a large body size and of a predominantly frugivorous and opportunistic diet.