The effects of the plant essential oil obtained from yarrow (Achillea millefolium)were evaluated against planktonic cells and biofilms of Listeria monocytogenes and L. innocua isolates obtained from food processing environments. The essential oil was found to have strong antimicrobial activity against planktonic cells which was found to be equivalent to that of tea tree oil (Melaleuca alternifolia). Crystal violet (CV) and 3-[4, 5-dimethylthiazol-2-yl]-2, 5- diphenyltetrazolium bromide (MTT) reduction assays were performed to assess the ability of yarrow essential oil to inhibit cell attachment and metabolic activity of biofilms, respectively. CV assays indicated that yarrow essential oil was most effective in inhibiting initial cell attachment on polystyrene, stainless steel and high density polyethylene surfaces, even at sub-MIC levels, but was less inhibitory towards preformed (6 hour) biofilms formed on polystyrene. MTT assays showed that the essential oil was able to significantly inhibit the metabolic activity of biofilms. Gas chromatography-mass spectrometry of yarrow essential oil identified several monoterpenes (b-pinene, 1,8-cineole, terpinene-4-ol and caryophyllene) as major components. Two of these (b-pinene and 1,8-cineole) have been previously reported to exhibit anti-L. monocytogenes activity. Considering the emergence of antibiotic resistant bacteria and the inclination of consumers towards natural alternatives to chemical sanitizers, further characterization of the bioactive compounds in yarrow essential oil would be useful for developing novel disinfectant or sanitizer formulations for application in the food industry.