Unusually long pyrimidine tracts (polypyrimidines), ranging from 100 to over 1000 nucleotides in length, have been found in Drosophila melanogaster DNA. They are compared to shorter pyrimidine tracts (25-150 nucleotides) which have previously been found in L-cell DNA. Both species were able to anneal to homologous DNA; Drosophila polypyrimidines formed stable hybrids while L-cell polypyrimidines formed hybrids of lower thermal stability. In both cases, the kinetics of the reaction was rapid, suggesting that these tracts are part of highly repeated DNA.