Cultures of Bacillus subtilis lysogenic for the temperature bacteriophage SP beta release "betacin", a bacteriocinlike substance that inhibits B. subtilis strains which do not carry this phage. Production of betacin is blocked by mutations in the bet gene on the prophage and a second phage gene, tol, is apparently involved in making the lysogen itself tolerant to betacin. Mutations in a bacterial gene betR, located on the B. subtilis chromosome between metC and pyrD, render nonlysogens tolerant to betacin.