Clostridium botulinum is not a well-defined species of bacterium. Instead, it is a conglomerate of four culturally distinct groups of organisms that, among them, produce seven serologically distinct toxins, all with similar pharmacological action. The principal habitat of C. botulinum is the soil, although its distribution in the soil is sometimes highly regional. Infant botulism is caused by two types of C. botulinum: type A and the proteolytic strains of type B. Type A strains, to whose toxin humans seem most susceptible, are found most frequently in the soil of the western United States; type B strains are somewhat more universally distributed, with a higher frequency of isolation from the soil of some Appalachian areas. The frequency of occurrence of type A and type B food-borne botulism parallels the distribution of these types in the soil.