The heterochromatin of the Chinese hamster sex chromosomes was analyzed by different banding techniques. Combined results obtained after differential Ba(OH)2 treatment, BrdU incorporation, Giemsa 11, and staining with quinacrine permitted the characterization of different regions in the heterochromatic portions of the X and Y chromosomes. In the light of these observations, the chiasma observed in the sex bivalent of Chinese hamster spermatocytes was localized within specific heterochromatic regions. The homologous segments consist of the entire short arm of the Y and the distal end of the long arm of the X up to band q21. These regions are probably not rich in highly repetitive DNA sequences, which are more resistant to alkali denaturation, or in satellite DNA, which is stained by Giemsa 11. Thus the heterochromatin in the homologous regions of the sex chromosomes allows the formation of a chiasma. The heterogeneity found in these heterochromatic regions may help to establish a more precise relationship between heterochromatin and recombination.