MAMMALIAN constitutive heterochromatin probably occurs in a variety of forms but chiefly two modes1. These are (a) centromeric, which is found chiefly at the centromeres and paracentric constrictions, and (b) intercalary, which is found in other chromosomal regions. Our experience with the banding techniques has led us to believe that the G bands (so called by a convention adopted by the Fourth Congress of Genetics, Paris, 1971) contain a considerable complement of intercalary constitutive heterochromatin. We propose that it is chiefly this latter type of A–T rich constitutive heterochromatin that is enriched at the sites of the G bands, and that in the G banding techniques Giemsa stains the protein remaining at these sites after the various pretreatments.