SUMMARYColiphage BA14 was isolated from sewage and shown to be related to phages T7 and T3. It is similar to T3 in that it directs the synthesis of an S-adenosylmethionine-cleaving enzyme (SAMase) early upon infection. However, it differs from all other known T7-related coliphages by the inability of its RNA polymerase (gene 1 product) to transcribe T7 DNA or T3 DNA. BA14, T7 and T3 also show marked differences in ~ autoradiographic patterns of their gel-electrophoreticalty separated 35S-labelled intracellular phage proteins, restriction endonuclease HpaI cleavage patterns of their DNAs, and serological specificities of their infectious particles. Other distinctive features became apparent upon simultaneous mixed infection with BA14 and T7 or T3: inability of BA14 to produce genetic recombinants with either T7 or T3; lack of functional complementation between amber mutants of BA14 and T7 or T3; mutual exclusion and depression of the burst size of the mixedly infected cells.