Generalized transducing phage similar to phage P1 in Escherichia coli was isolated from E. coli W39, an antigenic test strain of the O121 group. This phage, designated φw39, was reciprocally heteroimmune to phages P1 and P7, but nonreciprocally heteroimmune to phage D6. Transduction experiments using various R plasmids with different molecular weights suggested that phage φw39 could transduce at least 65 megadaltons DNA. As in the case of P1 prophage, φw39 prophage existed as a plasmid belonging to incompatibility group Y and carried a dnaB-like function. The molecular weight of φw39 plasmid was nearly the same as that of plasmid, i.e., 58.6 megadaltons. Despite the pronounced structural and functional similarity of phages φw39 and P1, restriction cleavage patterns of their genomes differed considerably.