Numerous essential oils from plants belonging to the family Lamiaceae and commonly used infood industries, cosmetics, aromatherapy, etc. have shown to have significant pesticide properties. Speciesbelonging to the genus Thymus show a number of variations in the chemical composition of secondaryproducts. These variations represent an opportunity to study the ecological role of these essential oils. In this paper the authors investigate the chemical composition of essential oils from seven populations of Thymus balcanus, in order to identify natural populations with a high pesticide potential. The vegetal material was collected during flowering period from the Făgăraş Mountains, from different altitudes (1100-2000 m). The essential oil has been extracted using a NeoClevenger apparatus and the separation and the identification of the components have been carried out using a GC-MS Agilant 6890 N with a spectrometric mass detector 5973 and an auto sampler. Significant essential oil polymorphism was found in the chemical composition of volatile oils of Thymus balcanus, probably representing a way of adapting to different habitat conditions.Thus, at hight altitude the main constituent of essential oil was found to be thymol (23.60%), followed bylinalool (17.63%) and linalool acetate (16.12%) and at low altitude the main constituent is farnesol, followedby germacrene D, β-caryophyllene, thymol, carvacrol, estragol and linalool.