Nonrandom associations between the short arms of the sex chromosomes of six Trichosurus vulpecula were studied in 455 cells from the spleen and bone marrow. The association occurs in both sexes and was found to be independent of age. There was, however, a significant difference in the degree of association between the two tissues studied. Regions of constitutive heterochromatin were found to be located in the short arms of the X and Y chromosomes by using a C-banding technique. Nucleolar organizers were detected in the middle of the long arm of the X chromosome and in the short arm of the Y chromosome by N-banding. These results indicate that the nonrandom association of the sex chromosomes may reflect an association between heterochromatin regions during interphase of the cell cycle.