Technical Abstract: Escherichia coli O157: H7 and Salmonella Typhimurium DT104 are important food borne pathogens affecting the beef and dairy industries and strategies are sought to rid these from cattle at slaughter. Both pathogens possess respiratory nitrate reductase, long known to also reduce chlorate to the lethal chlorite ion. Since most anaerobes lack respiratory nitrate reductase, we hypothesized that chlorate may selectively kill Escherichia coli O157:H7 and Salmonella Typhimurium DT104 but not potentially beneficial anaerobes. In support of this hypothesis, we found that concentrations of Escherichia coli O157: H7 and Salmonella Typhimurium DT104 were reduced from approximately 1,000,000 colony forming units (CFU) to below our level of detection (<10 CFU) following in vitro incubation (24 hr) in buffered ruminal contents (pH 6.8) containing 5 mM added chlorate. In contrast, chlorate had little effect on the most probable number (mean +/- SD) of total culturable anaerobes (ranging from 9.9 +/- 0.72 to 10.7 +/- 0.01 log base 10 cells/ml). Thus, chlorate was bactericidal to E. coli O157: H7 and Salmonella Typhimurium DT104 but not to potentially beneficial bacteria. The bactericidal effect of chlorate was concentration dependent (less at 1.25 mM) and markedly effected by pH (more bactericidal at pH 6.8 than pH 5.6).