Q- and G-banding studies of Aotus trivirgatus trivirgatus and Aotus trivirgatus griseimembra are presented. After the analysis of 8 specimens, 2 A. t. trivirgatus and 6 A. t. griseimembra and the review of the literature on the field we state the existence of two different karyotypes, 2n = 54 and 2n = 50, for A.t.trivirgatus and three different karyotypes, 2n = 54, 2n = 52 and 2n = 50, for A. t. griseimembra. It was found that Robertsonian translocations were the mechanism involved in the numerical reduction of the chromosomes. The data presented include the statement of two natural hybrids and the cross-breedings which originated them.