Chromosome studies were performed in a male representative of Cacajao melanocephalus. The diploid number was 45: the chromosome complement consisting of 12 pairs of acrocentric autosomes, 9 pairs of biarmed autosomes and 1 heteromorphic pair composed of one subtelocentric and one acrocentric chromosome. The X chromosome was submetacentric and the Y chromosome was apparently translocated to an acrocentric autosome. G-banding patterns were studied and the results compared to karyotypes previously described for Cacajao rubicundus and Cacajao calvus.