An analysis of the meiotic chromosomes of Cebus apella (Cebidae, Platyrrhini) has allowed for the visualization of constitutive heterochromatic regions throughout the meiotic cycle and has permitted the identification of four chromosomes in the first and the second divisions. In pachytene nuclei, and in pachytene spreads where synaptonemal complexes were visible, one heterochromatic region was frequently unpaired. No chiasmata were found in heterochromatic regions, and there was no evidence of chiasma terminalization. The role of heterochromatin in homologous pairing and its relationship to chiasma position and number in meiotic configurations were discussed. A comparison was made between bivalent 9 in C. apella and bivalent 21 in man, based on the interspecific homologies between these chromosomes. A similar comparison was made between the Algerian and Asian hedgehogs, where marked differences in the content of constitutive heterochromatin occur in two chromosome pairs.