Previous reports indicated that some competing bacterial strains produce antimicrobials that can reduce E. coli O157:H7 cell numbers in cattle (17, 26, 37). Our results from treating cattle drinking water with competing E. coli revealed that growth of E. coli O157:H7 was controlled, but the treatment had minimal effect on reducing E. coli O157:H7 populations. Hence, it is not a practical approach for treating drinking water to control the pathogen.