ABSTRACT. We described the karyotypes detected in 23 Aotus specimens captured on Southwestern Amazon region (63~ 9~ This region should be occupied by A. nigriceps. However, the animals described here have a fur pattern and a karyotypes similar to those of A. azarae boliviensis and A. infulatus. The females presented 50 chromosomes and the males 49 as a consequence of fusion between the chromosome Y and an autosome. This sample presented polymorphisms of G-and Cbanding and NOR-staining. These animals are karyotypically intermediate between A. a. boliviensis and A. infulatus. The taxonomy and karyotypic evolution of these taxa are discussed.