Population densities of two species of night (or owl) monkeys (Aotus nancymai and Aotus vociferans) were estimated using transect census methods. Densities of Aotus nancymai were approximately 46.3 individuals/km2 in lowland forests and 24.2 individuals/km2 in highland forests. For Aotus vociferans densities were 33.0 individuals/km2 in lowland forests and 7.9 individuals/km2 in highland forests. A. vociferans occurs north of the Río Amazonas‐Marañon and A. nancymai south of it, except for the northern enclave between the Ríos Tigre and Pastaza. The two species are nowhere sympatric. However, the two known karyotypic forms of A. vociferans occupy the same habitats throughout the Peruvian range of the species. Copyright © 1988 Wiley‐Liss, Inc., A Wiley Company