A program to analyse the length and frequency distribution of specific base tracts in genomic sequences is described. The frequency of oligopurine.oligopyrimidine tracts (R.Y. tracts) in a data base of 163 transcribed genes is analysed and compared. The complete genomes of SV40 virus, N. tobacum chloroplast, yeast 2 micron plasmid, bacteriophage lambda, plasmid pBR322 and the E. coli lac operon are also analyzed. A highly significant overrepresentation of oligopurine and oligopyrimidine tracts is observed in all eukaryotic genes examined, as well as in the chloroplast genome. The overrepresentation is evident in all gene subregions of the chloroplast, in the following order: intergenic regions, 3' downstream and 5' upstream (promoter), 5' and 3' untranslated, introns and coding regions. In genes coding for basic proteins, oligopurine rather than oligopyrimidine tracts are found on the coding stand. In prokaryotic genes only the longest R.Y. tracts (greater than or equal to 12) are found in excess, and are concentrated near regulatory regions. While a structural role for R.Y. tracts is most likely in intergenic regions, a functional role, as initiation sites for strand separation, is proposed for regulatory gene regions.