Forty species of anaerobes were screened for the ability to produce an ether-extractable mutagen which is present in the feces of 15 to 20% of individuals in populations at high risk for colon cancer. This mutagen can be produced in vitro by incubating the feces of these individuals anaerobically or by supplementing anaerobic broths with methanol extracts of the feces and incubating them with a dilute fecal inoculum. Of the anaerobes screened, strains of five species of Bacteroides (B. thetaiotaomicron, B. fragilis, B. ovatus, B. uniformis, and Bacteroides group 3452A) were capable of producing five- to eightfold increases in the concentration of mutagen. For in vitro production in broth, all producers required bile and the methanol extract for feces from a person who excretes the mutagen. Mutagen production appeared to be constitutive and occurred during the stationary phase of growth. Cell-free extracts were active and produced mutagen considerably faster than did whole cells. Our observations indicate that the excretion of this mutagen by certain people is dependent on the presence of some precursor of unknown origin. The mutagen-producing species of bacteria are among the most common of the intestinal microflora and were present in mutagen excreters and nonexcreters as well.