This eighteen-month study focuses on the distribution, ecology and systematics of leaf monkey species especially P. hosei in Sarawak. This is the first detailed study ever conducted on the nominotypical P. hosei. The range of distribution of leaf monkeys has shrunk because of habitat destruction and hunting. P. chrysomelas and P. hosei are extinct in Niah, Lambir Hills and many other areas. The occurrence of P. chrysomelas in Similajau and P. frontata in Mukah is unknown. P. hosei now occurs in fragmented populations in isolated forest patches in Bukit Nakan, Mt. Dulit, Sg. Dapui, Upper Baram, Usun Apau, and perhaps Lower Tinjar. P. rubicunda is reported on Mt. Dulit but its status in Upper Baram is unknown. Ecological data of P. hosei was collected in Sg. Unin and Sg. Dapui. P. hosei lives in groups of six to eight and eats mainly young leaves and unripe seeds. At Sg. Unin, P. hosei groups share the same home ranges due to the forest type; which is patchy because of logging, and suffers from rampant hunting, and has no major river. At Sg. Dapui, different groups inhabit different home ranges; here, a big river runs through the area with smaller tributaries creating natural boundaries, the forest area is bigger in size and there is lower level of habitat disturbance and hunting. P. hosei and rubicunda are sympatric on Mt. Dulit. Four different types of P. hosei calls were identified: loud call, inquisitive/warning call, alarm/flee call and intimate call. The loud call (male) has up to 33 audible exhalation sounds (>2 sec.) and a peak frequency of 2000-3000 Hz and 4-6 sub-units of calls. There are marked differences between the loud calls of P. hosei and P. sabana. Among the 'P. hosei group', there are significant differences in cranial characteristics, pelage colour and molecular phylogenetics but there are no differences between P. hosei and the putative P. everetti which should be considered as the female form of P. hosei. P. hosei, P. sabana and P. canicrus should be distinguished at the species level. Without serious conservation measures, I predict that the Presbytis species in Sarawak will be extinct in the following order: P. chrysomelas (already extinct in the east of the state), followed by P. frontata, P. hosei and finally P. rubicunda
