Proboscis monkey (Nasalis larvatus) social organization was studied at the Natai Lengkuas station, Tanjung Puting National Park in Kalimantan Tengah, Indonesia. Data were collected on ten one-male groups and two all-male groups using scan sampling and event sampling during follows, and through evening census surveys. Proboscis monkeys were found to form stable one-male groups. Specific groups associated at their sleeping sites, in a manner similar to the fission-fusion pattern described for hamadryas and gelada baboons. Analyses performed on these associations indicated that the groups formed two separate bands. These separate bands appear to be a secondary level of organization, distinct from the primary level of organization, the one-male and all-male groups. This is the first evidence for such a two-tiered system of organization in an arboreal colobine species. Proboscis monkeys closely resemble gelada baboons in their organizational pattern. In both species, adult females direct the majority of their affiliative behavior towards their offspring and other adult females, males do not engage in active herding, all-male groups are present, and one-male groups do spend time apart from other groups. Home ranges of both groups and of bands of proboscis monkeys overlapped extensively; temporal avoidance between bands may have been mediated by early morning vocalizations and branch shaking displays.