Distribution of purine and pyrimidine residues in the T-rich fragments isolated from bacteriophage f1 DNA was studied. The T-rich fragments as a whole consisted of about 275 nucleotides and contained 45–49 molar percent thymidyli acid in contrast to the native f1 DNA which consisted of about 5, 500 nucletides and contained 34 molar per cent of thymidylic acid. 1. The native f 1 DNA and the T-rich fragments were apurinated in a mixture of formic acid and diphenylamine, and the products were fractionated first on a DEAE-Sephadex A-25 column according to their chain length and then on a DEAE-cellulose column according to their base compositions. The results indicated that the native f 1 DNA contained 15 thymidine tetranucleotides, 5 thymidine pentanucleotides, and 1 thymidine hexanucleotide. Polydeoxycytidylic acid runs were found less frequently. The T-rich fragments contained 1 thymidine pentanucleotide, 1 thymidine hexanucleotide, and several pyrimidine oligonucleotides rich in thymidine residues. 2. The ultraviolet-irradiated T-rich fragments were more resistant to snake venom phosphodiesterase [EC 3.1.4.1] than the ultraviolet-irradiated fragments which were prepared from f 1 DNA by sonication and had almost equal molecular size. 3. The T-rich fragments formed hybrids with synthetic polyribouridylie acid, but not with polyribocytidylic acid, suggesting that the fragments might contain several numbers of deoxyadenosine-rich sequences. 4. On these basis and also from the observation that the T-rich fragments exhibited thermal denaturability and high degree of renaturability, it was considered that a portion of the T-rich fragments might be hydrogen-bonded, in a single polydeoxy-nucleotide chain, between the thymidylic acid runs and the deoxyadenylic acid runs. 5. The native f 1 DNA also exhibits thermal denaturability and renaturability. The DNA may have a folded structure near the pyrimidine-rich region.