The karyotypes of three platyrrhine monkeys (Callithrix jacchus, Saimiri sciureus, Aotus trivirgatus) were studied using a high resolution banding technique. Their chromosomes were compared with those of man and of Lagothrix lagotricha and Cebus capucinus, previously analysed. Based on the numerous banding homoeologies, a reconstruction of the karyotype of their common ancestor is proposed.