The aim of this study was to determine the antimicrobial activity of six medium-chain fatty acids (MCFA) against enteropathogenic Escherichia coli strain C6 and compare it with that of citric, lactic and fumaric acid at different pH. MCFA were better antimicrobials than other organic acids; the antimicrobial activity of fatty acids toward the C6 strain was pH-dependent. Beneficial effects of citric, lactic and fumaric acid reported by animal nutritionists are probably related to factors other than theirdirect antimicrobial action.