Freshly isolated DNA of phage PSP3, whose morphology closely resembles that of phage P2, contained both circular and linear molecules about 31 kb in length. Linear PSP3 DNA molecules possess single-stranded cohesive termini (cos). Sequencing of the fragment anticipated to contain cos revealed a 19-base sequence identical to cos of phage 186. Of the 107 bp to the right of cos, 94 were identical in 186 DNA (88% similarity), and of the 370 bp to the left, 229 were identical (62% similarity). Cos flanking sequences in both P2 and P4 were also highly conserved in PSP3. A number of restriction sites were at similar locations on the two phage DNAs. The parasitic phage P4 propagated on PSP3 lysogens. PSP3 integrates into the Escherichia coli chromosome at 27 min.