The purpose of this study was to examine the chemical composition and in vitro bioactivity of the essential oils isolated from five Thymus species and cultivars (T. vulgaris, T. vulgaris cv. Faustini, T. citriodorus, T. citriodorus cv. variegata, T. calcareus). Chemical composition was investigated by GC-MS and GC-FID. Antioxidant potential was evaluated by free radical scavenging and reducing power assays [1]. Comet assay was used to detect the genotoxic/antigenotoxic effects of thyme essential oils [2]. AntiAspergillus flavus, aflatoxin-producing fungus and anti-tyrosinase activities were also investigated [3]. The study revealed significant differences in chemical composition and bioactivity among Thymus species and cultivars. Thymol (55.4 and 55.45%) was identified as major constituent in T. vulgaris and T. calcareus essential oils, respectively. Lavandulol (54.27%) was the major constituent of T. citriodorus essential oil whereas geraniol (22.12 and 38.28%) was found to predominate in essential oils isolated from T. vulgaris cv. Faustini and T. citriodorus cv. variegata, respectively. T. vulgaris and T. calcareus essential oils showed significant anti-Aspergillus flavus (MIC = 0.25 μL/mL, MFC = 0.5 μL/mL) and antioxidant effects (EC50 = 0.15 and 0.37 μg/mL, respectively in DPPH assay, EC50 = 0.04 and 0.05 μg/mL, respectively in reducing power assay). At 25 μg/mL, T. vulgaris essential oil was more efficient than T. calcareus essential oil in protecting DNA against H2O2-induced oxidative damage (6.21 and 5.52% vs. 25.13 and 7.26% tail DNA in H2O2 pre- and post-treatment protocols, respectively). At 50 μg/mL, T. citriodorus essential oil inhibited tyrosinase activity by 35.75% whereas kojic acid, a wellknown tyrosinase inhibitor, exhibited 42.85% inhibition.