Infant botulism is an age-dependent illness that is caused by the toxin produced by Clostridium botulinum infecting the intestinal tract. Because the composition of the intestinal microflora determines the resistance of mice to enteric colonization by C. botulinum, attempts were made to identify the kinds of bacteria that prevent this in vivo growth. Orogastric challenges of 50 spores of C. botulinum type A were given to adult germfree mice, which are highly susceptible, and to gnotobiotic adult mice carrying the eight species comprising the Charles River Altered Schaedler flora or two or three of the limited number of species constituting a different flora (the University of Wisconsin Gnotobiote Laboratory [UW-GL] flora). These floras did not prevent infection due to C. botulinum; however, death rates among the mice with defined floras were significantly lower than those among germfree mice exposed to C. botulinum. Botulinum toxin continued to be produced while animals surviving nearly lethal cases of botulism convalesced slowly but uneventfully. Gnotobiotic mice with the complete UW-GL flora were not infected when challenged with 105 spores of C. botulinum.