The content of 5-methylcytosine (5MC) and 6-methyladenine (6MA) in modified and nonmodified DNAs from B. subtilis and B. subtilis phage SPP1 were determined. Nonmodified SPP1 · O DNA contains about 15 5MC residues/molecule. Each modified SPP1 ·R DNA molecule carries 190 modification specific methyl groups. This number is sufficient to account for modification of the 80 restriction sites in SPP1 DNA (Bron and Murray, 1975) against endo R · Bsu R, assuming each modified site contains two 5MC residues. Resistance of SP01 DNA against endo R · Bsu R restriction both in vivo and in vitro is probably not due to methylation of endo R·Bsu R recognition sites.