We investigated the role of the rumen fermentation as a barrier to the foodborne pathogen, Escherichia coli O157:H7. Strains of E. coli, including several isolates of O157:H7, grew poorly in media which simulated the ruminal environment of a well-fed animal. Strains of E. coli O157:H7 did not display a superior tolerance to ruminal conditions which may facilitate their colonization of the bovine digestive tract. Unrestricted growth of E. coli was observed in rumen fluid collected from fasted cattle. Growth was inhibited by rumen fluid collected from well-fed animals. Well-fed animals appear less likely to become reservoirs for pathogenic E. coli. These results have implications for cattle slaughter practices and epidemiological studies of E. coli O157:H7.