Studies of susceptibility to the FCB-1 strain of Plasmodium falciparum from Colombia were conducted in three different groups of Aotus monkeys. The results indicated that Aotus trivirgatus griseimembra from the north of the Magdalena Valley were the most susceptible host. A. t. griseimembra from the central Magdalena Valley appeared to be less susceptible to infection, while Aotus monkeys from the eastern Andes were even more resistant to infection. Serum electrophoretic studies were not useful as indicators of resistance or susceptibility to infection. However, the presence of 58 chromosomes in monkeys from the east Andes, a karyotype not previously reported, may be an indicator for the resistance of these animals to the FCB-1 strain. Based on these results, it is possible to assume that Aotus monkeys from the eastern Andes region of Colombia may represent a new species of Aotus; further studies are needed to clarify its taxonomic status.