Clostridium novyi type A, the cause of gas gangrene in man and animal, produces four main toxic components (alpha, gamma, delta, and epsilon). The alpha toxin is a necrotizing toxin produced in lethal amounts by strains of type A. This lethal toxin induces a characteristic subcutaneous colorless gelatineous edema when guinea pigs are infected with C. novyi type A. In this paper we report further evidence on the relationship of bacteriophages to the production of alpha toxin by C. novyi type A. The results showed that five phage-sensitive bacterial strains that did not produce alpha toxin could be isolated from alpha toxin producing stains. These nontoxigenic strains could be infected with specific bacteriophages isolated from the different toxigenic strains of C. novyi type A and converted to alpha toxin production. The toxigenicity of these converted strains depended upon the continued participation of specific bacteriophages. These bacteriophages were shown to be very unstable in culture supernatant fluids.