Aims: Chlorate kills Escherichia coli O157:H7 and may be an effective feed additive for use in food animals. This study was designed to determine if development of chlorate-resistant E. coli O157:H7 strains was likely. Methods and Results: Escherichia coli O157:H7 was chlorate-sensitive, but became chlorate-resistant in pure batch culture and in sterilized faecal fluid; it was killed in mixed culture and did not become resistant. Addition of chlorate to continuous pure cultures caused chlorate resistance, but chlorate addition to mixed continuous culture eliminated E. coli O157:H7 and no resistance occurred. Piglets challenged with E. coli O157:H7 were treated with chlorate; populations were reduced and colonies were always chlorate-sensitive. Conclusions: Chlorate-resistant E. coli O157:H7 can be selected in pure, but not mixed culture, and results suggest that terminal chlorate feeding will not select for chlorate-resistance in vivo. Significance and Impact of the Study: Chlorate can reduce food-borne pathogens prior to harvest, but development of resistance does not appear likely in vivo.