In a synchronized culture of E. coli K12, prophage P1 DNA did not replicate at a specific stage in a cell cycle. It was shown that prophage P1 DNA is able to replicate independently of the initiation of the host mutated in N167. The presence of one copy of P1 DNA per bacterial chromosome was generally observed. The ratio of P1 DNA to bacterial DNA increased up to 4 copies per bacterial chromosome, when the initiation of the replication of the chromosomal DNA was inhibited.