Ajowan is a popular spice that is used in folk medicine in Algeria. Most studies reported that thymol is the main component of the seed oil. Isothymol was reported to be a constituent of essential oil isolated from aerial parts. Using plants harvested in five different locations in north western Algeria, every two weeks during the flowering stage, we demonstrated that isothymol was the major component when the plant was collected near the sea at low altitude and at the beginning of the flowering stage. In other cases, thymol is the predominant constituent.