Distribution and demography of all diurnal primates were studied in Silent Valley National Park and adjacent areas for a period of three years from 1993 to 1996. Fourteen troops of lion-tailed macaque, eighty-five troops of Nilgiri langur, fifteen troops of bonnet macaque and seven troops of Hanuman langur were observed. Of these, the Nilgiri langur was randomly distributed, whereas the lion-tailed macaque troops were confined to the southern sector of the Park. Bonnet macaques and Hanuman langurs were occasional visitors. The Silent Valley forest remains one of the most undisturbed viable habitats left for the endemic and endangered primates lion-tailed macaque and Nilgiri langur.