Coliphage BF23 develops in Salmonella typhimurium rough strains. The phage is neither restricted nor modified by S. typhimurium. The growth patterns of the phage were slightly different in S. typhimurium than in Escherichia coli, although phage propagated on S. typhimurium is identical to the phage propagated in E. coli by several criteria used. Mutants of S. typhimurium resistant to BF23 were isolated and found to map (by P22- and Pl-mediated transduction) in the same position as bfe mutants of E. coli. The order of genes was: metB - argC - bfe - rif - purD - metA. Phage BF23 does not form plaques on smooth S. typhimurium strains, since the phage fails to adsorb irreversibly to smooth cells. Nevertheless, on solid agar, the phage prevents growth of many (but not all) smooth strains. Moreover, UV- and alkali-inactivated phage BF23, although unable to form plaques on sensitive hosts, retains the ability to prevent growth of the host on solid medium. This ability is sensitive to protease and resistant to DNAse and RNase. Heat treatment of the phage causes rapid loss of the cell-growth-preventing-ability whereas the ability to form plaques is lost much more slowly. These results lead to a proposal that phage BF23 virions carry a colicin-like factor that kills sensitive cells.