The karyotypes of 22 South American owl monkeys (Aotus trivirgatus) are surveyed. Animals were assigned on the basis of coat color variation to two previously described subspecies, Aotus trivirgatus griseimembra and Aotus trivirgatus trivirgatus. A Robertsonian polymorphism, distributed consistent with a Hardy-Weinberg equi librium, was found among 17 Aotus trivirgatus griseimembra, whose diploid numbers were 54, 53, and 52. The 5 Aotus trivirgatus trivirgatus all had diploid numbers of 54. Their karyotypes are significantly different from those of Aotus trivirgatus griseimembra, however, and may indicate the need for taxonomic re-classification.