We report information on population density, group size, and habitat preferences of primates along the lower Río Urubamba and in the Río Urubamba–Río Tambo interfluvium, in central–eastern Peruvian Amazonia, an area that has been little explored with regard to its primate fauna. During 425 km of transect walks in October–November 2008 and April–May 2009 totally 174 groups of nine primate species were encountered, the most common being Callicebus brunneus (45 groups), Saguinus imperator (41 groups), and Aotus nigriceps (26 groups). Group sizes were smallest for A. nigriceps and C. brunneus (mean of 2.8 and 2.9, respectively) and largest for Saimiri boliviensis (mean 15.6). Population densities were lowest for Lagothrix cana (3.3 individuals/km 2) and highest for A. nigriceps (31.1 individuals/km 2). Groups of C. brunneus, S. imperator, S. boliviensis, Cebus albifrons, and Cebus apella were most frequently (83 % of sightings) encountered in semi-dense or in open primary forest that included stands of bamboo ( Guadua sarcocarpa) or where bamboo was a very common species.