The purpose of this study was to investigate the primary mode of action of Cistus ladaniferus essential oil active fractions on Staphylococcus aureus strain ATCC6538P (CIP 53.156). The mode of inhibition of the active fractions was assessed by determining the minimum inhibitory concentration (MIC). The effects of time on cell integrity were determined by time-kill, bacteriolysis and loss of 260 and 280-nm-absorbing material assays. Measurement of intra-and extracellular ATP was used to evaluate the energy remaining in the cells after treatment. A bacteriostatic and a bac-tericidal mode of inhibition were established respectively for acetate and alcohol fractions at their MIC. No intracellular material leakage and no lysis occurred after treatments with these fractions. In both cases, we observed a decrease of the ATP level within S. aureus cells whilst there was no proportional increase outside the cells. However, the effects induced by alcohols are more pronounced than those provoked by acetates. Indeed, marked structural changes were observed by transmission electron microscopy (TEM). The septal material of cells undergoing division became thicker and stained more lightly. The proportion of septa is also markedly increased and defective with respect to placement. These observations suggest a blocking in cell division, probably caused by the inhibition of ATPase or a disturbance in proton motrice force by the hydrophobic molecules viridiflorol and ledol, mainly present in alcohol fraction.