DNA isolated from Drosophila melanogaster embryos contains measurable amounts of dAT, the copolymer of deoxyadenylate and deoxythymidylate. In wild-type (XX,XY) embryos, dAT constitutes 4% of the total DNA. In embryos containing extra Y chromosomes, or even extra portions of Y chromosomes, dAT constitutes as much as 7% of the total. The enrichment for dAT is dependent upon the number of extra short arms of the Y chromosome (YS) or long arms (YL) of the Y chromosome per individual. This enrichment of dAT dependent on the Y chromosome may be interpreted in several ways. The most straightforward interpretation is that dAT is present in high concentrations on the Y chromosome.