Thirty-three strains ofClostridium perfringenswere screened for bacteriocin production. Four bacteriocin-producing strains were detected by plating the supernatant fluids of these cultures on all available strains ofC.perfringensseeded in semisolid agar and noting zones of bacterial inhibition after subsequent incubation. The spectrum of sensitive strains differed for each bacteriocin as did the degree of bacterial sensitivity to each bacteriocin.One bacteriocin and one indicator strain were chosen for further study. This bacteriocin, which was spontaneously produced during the logarithmic growth phase of the bacteriocinogenic strain, was not inducible with ultraviolet light but was sensitive to heat and trypsin. Adsorption of bacteriocin to the indicator strain was not detected and electron microscopy did not reveal any particulate substance associated with bacteriocin activity. The degree of bacterial inhibition was dependent on the titer of the bacteriocin used, and the age of the indicator culture appeared to influence its relative response to bacteriocin treatment.