Technical Abstract: Enterohemorrhagic Escherichia coli are important foodborne pathogens that cause potentially fatal disease in humans. These pathogens, like most members of the family Enterobacteriaceae, possess the ability to respire anaerobically using nitrate as an electron acceptor. Since most, if not all, known respiratory nitrate reductases coincidentally reduce chlorate intracellularly to the toxic chlorite ion, a study was performed to assess the effect of intraruminal sodium chlorate (0.2 g/kg body wt) administration on E. coli in the bovine gut. Four ruminally cannulated cows were allocated to a 4 x 4 Latin square design, with a factorial arrangement of treatments (chlorate treatment versus no treatment) and feeding regimes (fed versus fasted), to test for effects on gut concentrations of E. coli, total culturable anaerobes and on fermentation parameters. In support of the hypothesis that chlorate is bactericidal to E. coli but not potentially beneficial bacteria, concentrations of E. coli, but not total culturable anaerobes, were lower (P < 0.05) 10 and 24 h post chlorate administration, respectively, in rumen contents and feces of chlorate treated cows than in untreated cows. Fasting had little effect on ruminal or fecal E. coli concentrations and did not affect the bactericidal effect of chlorate. Chlorate treatment had little or no effect of fermentation efficiency, as evidenced by pH and volatile fatty acid production, and had no observable adverse effects on any of the cows. These results suggest that chlorate supplementation may be useful in the preharvest control of enteric pathogens.