Transect surveys were carried out in seven western Amazonian upland forest sites and compared with four additional sites to examine effects of hunting by humans on the structure of species-rich primate communities. Primate body mass was a strong positive correlate of its crude and metabolic population biomass in non-hunted but not in hunted sites. Primate body mass was a good negative correlate of population density in hunted but not in non-hunted sites. Group density was not clearly affected by hunting activity. Large primates had significantly lower group densities than small primates in both hunted and non-hunted sites. These trends are largely a consequence of differences in abundance of large-bodied genera (i.e.Alouatta, Ateles and Lagothrix), accounting for the bulk of the primate biomass in non-hunted sites, but being over-harvested or becoming extinct in sites hunted by man.