The island of Siberut in the Mentawai Archipelago, west of Sumatra, Indonesia, supports four of the five primate species endemic to the Archipelago, all categorized as threatened on the IUCN Red List. As a baseline for future research on the ecology and conservation of Siberut's primates we used a line-transect approach to survey primates in the Peleonan forest, site of the Siberut Conservation Project in northern Siberut. In 104 km of surveys we made a total of 391 observations of primates and estimated density and population sizes for the Project's 10.7 km2 main study site. Overall primate biomass density was estimated to be 697 kg km-2 and abundances, in individuals km-2, were: pig-tailed snub-nosed langur Simias concolor, 53.1; Siberut macaque Macaca siberu, 16.2; Kloss's gibbon Hylobates klossii, 8.9; Mentawai langur Presbytis potenziani, 8.2. These results confirm those from earlier studies that in undisturbed and unhunted primary forests S. concolor is more successful than the sympatrically occurring P. potenziani. Results for all species suggest considerable population sizes on the island and underline its importance for the conservation of Mentawai primates.