This study was conducted to evaluate the antibacterial activity of Inula graveolens and Santolina corsica essential oils on Staphylococcus aureus and investigate their effects at the cellular level. The mode of inhibition of both essential oils against S. aureus ATCC 6538P (CIP 53.156) was assessed by determining the minimum inhibitory concentration (MIC) and the minimum bactericidal concentration (MBC). The effects of time and treatment dose on cell viability were determined by time-kill and bacteriolysis assays. Marked structural changes were observed by transmission electron microscopy (TEM). A bactericidal mode of inhibition was established for both essential oils, which rapidly reduced the cell viability of S. aureus at their MIC (5 mg.ml(-1)). No lysis occurred after treatments with the MIC and eight times the MIC of each essential oil. Invaginations of the plasmic membrane with thickenings of the cell wall as well as an aggregation of the cytoplasmic contents were observed in S. aureus cells treated with the MIC of both essential oils. These results suggest that the cytoplasmic membrane and the cell wall are involved in the toxic action of Inula graveolens and Santolina corsica essential oils.