Resistance of Campylobacter to antibiotics is of significant public health concern. Identifying alternative non-antibiotic compounds that have antimicrobial activity against this genus is therefore important. The aim of this study was to establish the effectiveness and mode of action of gallic acid as an antimicrobial agent against Campylobacter jejuni and Campylobacter coli. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of gallic acid against eight antibiotic resistant C. jejuni and four C. coli strains was determined. Growth of all strains was inhibited by gallic acid (MIC 15.63ug/ml to 1000ug/ml). Gallic acid was only bactericidal towards two strains (both C. coli) at the concentrations studied (MBC 62.5ug/ml and 125ug/ml). A time kill assay was performed on the two C. coli affected by gallic acid and two other strains (one C. coli and one C. jejuni). The assay was performed with gallic acid in water (~pH3.84) and in phosphate buffered saline (PBS; pH 7.0) at the strains corresponding MIC. Inactivation occurred immediately for all four strains exposed to gallic acid in water. Inactivation occurred over 135 min for the two C. coli strains which gallic acid affected in PBS, while no effect on the other two strains was apparent at 180 min in PBS. No leakage of cellular content (absorbance at 260nm and 280nm) was apparent for any of the four strains at MIC. A change in colour of the solution of gallic acid in PBS from clear to turquoise was apparent for the two C. coli strains which gallic acid affected, but not the other two strains. This may be due to the removal of cations, such as calcium, from the cell outer membranes by gallic acid and may represent a non-pH dependent strain specific mode of action of this compound on Campylobacter.