Abstract The composition of the essential oil isolated by hydrodistillation from the aerial parts of Thymus fontanesii Boiss. et Reut., growing wild in Algeria (Tlemcen province) was determined by GC(RI), GC/MS and 13C-NMR. The oil was characterized by a high content of carvacrol (67â69%) and its antibacterial activity was tested against nine bacterial strains.