A search of sequence information in the GenBank flies shows that tracts of 15–30 contiguous purines are greatly overrepresented in all eukaryotlc species examined, ranging from yeast to human. Such an overabundance does not occur in prokaryotlc sequences. The large Increase in the number of oligopurine tracts cannot be explained as a simple consequence of base composition, nearest-neighbor frequencies, or the occurrence of an overabundance of oligoadenosine tracts. Oligopurine sequences have previously been shown to be versatile structural elements in DNA, capable of occuring in several alternate conformations. Thus the bias toward long oligopurine tracts in eukaryotic DNA may reflect the usefulness of these structurally versatile sequences in cell function.