The epidemiology of colitis induced by Clostridium difficile in hamsters was studied with a new bacteriophage and bacteriocin typing system. Fatal enterocolitis was induced by administration of N-formimidoyl thienamycin. Environmental cultures were obtained repeatedly throughout the experiments. Thirteen percent of 90 healthy hamsters were already colonized with C difficile on arrival from the supplier. Mortality from enterocolitis after antibiotic administration was 75% and was not diminished by use of a laminar-flow facility. The same uncommon bacteriocin type (83/1309/2329) of toxigenic C difficile that colonized hamsters on arrival was recovered from the cecal contents of all hamsters dying with enterocolitis and from most environmental isolates. Previously uncolonized , antibiotic-treated hamsters placed into cages where animals had died from enterocolitis also developed enterocolitis with the same bacteriocin type (83/1309/2329), an outcome suggesting acquisition of C difficile from the environment.