In human interphase nuclei containing Yq isochromosomes or two Y chromosomes, there are conspicuous somatic pairings between the brightly fluorescing Y-heterochromatin regions. These somatic pairings cannot be demonstrated in metaphases from conventional cell cultures because they are disrupted at the mitotic prophase. However, in lymphocytes cultivated in a medium containing distamycin A, the somatic pairing between Y-heterochromatin is preserved to metaphase. The present findings are compared with earlier observations of somatically paired heterochromatin. An explanation for the highly disparate frequency of somatic pairing between the Y-heterochromatin regions in Yq isochromosomes and YY chromosomes is proposed.