In a riverine forest along the Menanggul River, which is a tributary of the Kinabatangan River, Sabah, Malaysia, I observed an all-male group of proboscis monkey (Nasalis larvatus) consisting of 27-30 (mean: 28.8) individuals. This large size of the all-male group seems to be attributed to habitat fragmentation because of the expansion of oil palm plantations. A few females joined this all-male group. Sub-adult females copulated with subadult or large juvenile males. Since the mean male tenure period of this monkey was estimated to be longer than female maturity, and prematured females might leave their natal one-male groups to avoid inbreeding and temporarily participate in the all-male group where males were permissive to them. Even when females joined this group, no conflicts occurred among males.