AbstractThe essential oils from aerial parts of Ajania semnanensis Sonboli in two growing stages were isolated and their chemical constituents were identified. The oils were obtained by hydrodistillation and analyzed by gas chromatographyâmass spectrometry (GCâMS) and GCâflame ionization detector (GCâFID). Twenty-three and sixteen compounds comprising 96.5% and 92.0% of the essential oils were identified in the vegetative and flowering stages, respectively. The essential oils with the major compounds 1,8-cineole (32.5% and 27.4%), bornyl acetate (19.1% and 15.0%), camphor (18.9% and 12.2%), lavandulyl acetate (7.4% and 10.0%) and terpinen-4-ol (5.4% and 10.8%) were qualitatively similar. The antimicrobial activity of the oils was investigated against three Gram-positive and one Gram-negative bacteria and one fungus. The results showed a better activity of essential oils against fungus rather than the assessed bacteria.Keywords: Ajania semnanensisessential oilantimicrobial activityvegetative and flowering stageGCâMS