The karyotype of a male Cacajao calvus rubicundus (Platyrrhini) is described and compared with that of Cebus capucinus and that of Lagothrix lagotricha. The phyletic position of these three species is proposed. The existence of a translocation t(Y-autosome) is demonstrated in the specimen studied: the relationship between this translocation and the accumulation of heterochromatin on the rearranged element is discussed.