Very long runs of pyrimidine nucleotides (polypyrimidines), previously detected in DNA from Drosophila melanogaster, have now been localized to a "cryptic" satellite. These polypyrimidines have an average length of 750 nucleotides and account for about 3% of the thymine residues in total DNA. The buoyant density of the DNA component which contains the polypyrimidines was detected by centrifuging native DNA to equilibrium in a CsCl gradient, and then assaying each fraction for its content of polypyrimidines. A peak was detected at adensity of about 1.707 gm/cm-3, distinctly heavier than the main band of DNA (1.702 gm/cm-3). The buoyant density of polypyrimidine-containing molecules was little affected by differences in the molecular weight of the starting DNA in the range 10-5-10-7 daltons (single-stranded). Thus polypyrimidines (and their complementary polypurines) appear to form all or part of a "cryptic" satellite. Polypyrimidines have been isolated and characterized with respect to composition and buoyant density. Direct nucleoside analysis of unlabeled material indicated 34.5% deoxycylidine, 65.5% thymidine. Their banding position in neutral and alkaline CsCl gradients was consistent with a single-stranded DNA polymer of this composition.