Farnesol, a sesquiterpene alcohol with an aliphatic carbon chain, inhibited the growth of Staphylococcus aureus and induced the leakage of potassium ions. We investigated the action of farnesol on the cytoplasmic membrane of S. aureus. No ion channels that would account for the loss of potassium ions were detected. Electron paramagnetic resonance measurements suggested that farnesol proceeds into the cytoplasmic membrane of S. aureus cells, where it induces the disordering and eventual disruption of the cytoplasmic membrane. This was supported by the result that the effects of farnesol decreased by the addition of carotenoid which was the stabilizing reagent for the lipid bilayer.