One hundred and eighteen different strains ofProteus vulgaris were investigated for bacteriocinogeny. These and an additional 44 strains ofP. vulgaris were used as indicators. Sixty-seven of the strains had a nontransmissible killing effect on one or more of the indicator organisms and 30 of these 67 bacteriocins with different spectra of activity were further investigated. Individual bacteriocins killed from 5 to 87 of theP. vulgaris indicators and a number of 44 differentP. mirabilis strains but had no action on strains of other species of the familyEnterobacteriaceae. Broth cultures of bacteriocinogenic strains are inducible by ultraviolet light and yield bacterio- cin titres of about 1/100. Activity is sedimentable by high-speed centrifugation. Electron microscopy of all 30 preparations revealed similar phage-tail-like structures with a contractile sheath round a hollow core. The structures consisted of protein and did not contain DNA. The particles resembled some pyocins and also the tail of aP. vulgaris transducing phage. In 2 preparations a few phage-like particles resembling other Proteus phages were also seen. Bacteriocin activity was always associated with uncontracted sheaths, and triggered tails did not adsorb to susceptible organisms. We conclude that the tail-like structures are the products of defective lysogeny. The high incidence of the latter state may be accounted for by the selection of genes favourable to the host which were originally acquired through transduction by lysogenization or lysogenic conversion.