The Bacillus subtilis 168 chromosome was found to share extensive homology with the genome of bacteriophage phi 3T. At least three different regions of the bacterial genome hydridized to ribonucleic acid complementary to phi 3T deoxyribonucleic acid (DNA). The thymidylate synthetase gene, thyA, of B. subtilis and the sequences adjacent to it were shown to be homologous to the region in the phi 3T DNA containing the phage-encoded thymidylate synthetase gene, thyP3. SP beta, a temperate bacteriophage known to be integrated into the B. subtilis 168 chromosome, was demonstrated to be closely related to phi 3T. Other regions of the bacterial genome were also found to hybridize to the phi 3T probe. The nature and location of these sequences in the bacterial and phage chromosomes were not identified. It was shown however, that they were not homologous to either the thyP3 gene or the DNA surrounding the thyP3 gene. The chromosomes of other Bacillus species were also screened for the presence of phi 3T homologous sequences, and the thyP3 gene was localized in the linear genomes of phages phi 3T and rho 11 by heteroduplex mapping. It is suggested that the presence of sequences of phage origin in the B. subtilis 168 chromosome might contribute to the restructuring and evolution of the viral and bacterial DNAs.