DNA of P2 Hy 1 dis(large) , a phage believed to be genetically identical to Escherichia coli bacteriophage P2 except that the genes controlling and specifying immunity to superinfection have been replaced by foreign genetic elements, was mapped by the electron microscope heteroduplex method. It was found to have three regions nonhomologous to wild type P2; all were located on the right half of the molecule (the right half was defined as the A + T richer half as judged by electron microscope partial denaturation mapping). The nonhomologous regions were situated between 75.7 and 77.4%, 81.2 and 81.5%, and 88.3 and 102.2% from the left end of the P2 map. Moreover, around the 81% nonhomologous region there may be a partially homologous section. The replacement in the right end is unequal and greater than the wild type by about 0.28 μ m (2.2%). However, the cohesive ends of P2 Hy 1 dis(large) DNA were found to be similar to P2 DNA. Comparison with P2 lg cc vir 22, a deletion mutant of P2 phage, shows that the right end of the vir 22 deleted segment covers a part of the Hy dis replacement situated in the region 75.7–77.4%. Since, in both the phages, the immunity region is affected, this region can therefore be more closely located on the physical map. The genetic significance of the other two replacements in P2 Hy dis is not clear. It is interesting to note that the origin of replication of P2 DNA, which was located 89.0 ± 1.1% from the left end in a previous study, closely corresponds to the left end of the right most nonhomologous region in P2 Hy dis .