Abstract 1. 1. The frequencies of occurrence of purine and pyrimidine sequences have been determined in the DNA component associated with crystalline cytochrome b 2 ( l-lactate:cytochrome c oxidoreductase, EC 126.96.36.199) from baker's yeast and the results compared with those obtained from two preparations of high-molecular-weight yeast DNA and calf-thymus DNA. 2. 2. The purine sequences were obtained by the treatment of DNA with anhydrous hydrazine followed by alkaline hydrolysis and the pyrimidine sequences by treatment with diphenylamine in formic acid. The oligonucleotides in the digests were first separated and estimated on the basis of chain length by column chromatography on DEAE-cellulose. The individual components within each peak were then separated and estimated by means of low-voltage paper electrophoresis at pH 3.1. 3. 3. The results obtained have shown appreciable differences in the arrangement of purine and pyrimidine nucleotides in b 2- DNA , in whole yeast DNA, in DNA prepared from autolysed yeast and in calf-thymus DNA. It is concluded that the process of DNA degradation which leads to the formation of b 2- DNA is non-random.