This study was designed to examine the chemical composition of the essential oil and the in vitro antibacterial activities of the essential oil and methanol extract of Teucrium montanum. The inhibitory effects of the essential oil and the methanol extracts of Teucrium montanum were tested against thirteen bacterial species using the disc-diffusion method. GC/MS analyses revealed that the essential oil contained mainly sesquiterpenes, such as d-cadinene (17.19 %), b-selinene (8.16 %) and a-calacorene (4.97 %). The highest activities were obtained with the essential oil of Teucrium montanum against K. pneumoniae, B. subtilis, B. mycoides, E. cloaceae and A. chlorococcum. In addition, comparison of the antibacterial activities of the essential oil and the methanol extract showed that the essential oil exhibited the stronger antibacterial activities.