Thirty-one soil samples were examined for the presence of organisms capable of inhibiting growth and toxin production of strains of Clostridium botulinum type A. Such organisms were found in eight samples of soil. Inhibiting strains of C. perfringens were found in five samples, of C. sporogenes in three and of Bacillus cereus in three. Three of the C. perfringens strains produced an inhibitor effective on all 11 strains of C. botulinum type A against which they were tested, seven of eight proteolytic type B strains, one nonproteolytic type B strain, five of nine type E strains and all seven type F strains, whether proteolytic or nonproteolytic. They did not inhibit any of 26 type C strains, 6 type D strains, 4 type E strains, or 24 C. sporogenes strains. In mixed culture, an inhibitor strain of C. perfringens repressed growth and toxin production by a C. botulinum type A strain even though it was outnumbered by the latter about 40 times. It also repressed growth and toxin production of C. botulinum in mixed culture of soils in which this latter organism naturally occurred when cooked meat medium but not when trypticase medium was used.