Western Amazonia is one of the regions of the world with the highest terrestrial biodiversity. We conducted transect censuses between November and December 2012 in order to determine the diversity and the densities of primate populations, and their group sizes and habitat use in the Río Curaray region. During 610 km of transect surveys, we encountered 304 groups of 13 primate species. Woolly monkeys, Lagothrix poeppigii, were the most frequently observed (n = 49 sightings) and pygmy marmosets, Cebuella pygmaea, the least (n = 8). Population density was lowest for howler monkeys, Alouatta seniculus (9.8 individuals km-2) and saki monkeys, Pithecia aequatorialis (11.8 individuals km-2) and highest for squirrel monkeys, Saimiri macrodon (65.0 individuals km 2) and woolly monkeys (65.3 individuals km-2). Primate groups were most frequently encountered in "palmales de altura" (97 encounters of 12 species). In conclusion, the Río Curaray region harbors a very high diversity of primates, matching other sites in Amazonia and worldwide, and populations there are evidently healthy and well conserved. We recommend the creation of a protected area contiguous with the Yasuní National Park in Ecuador.