the composition of the essential oils hydrodistilled from the aerial parts of three population of tanacetum polycephalum. schultz-bip. subsp. polycephalum, collected from three different high natural altitude habitats; outskirts of behshahr, baladeh and savadkooh, in mazandaran province, north of iran, were analyzed both by gas chromatography (gc-fid) and gas chromatography-mass spectrometry (gc-ms). essential oil yield of dried plants obtained ranged from 0.76 to 1.3 % w/w. 41, 48 and 26 components were identified, representing 91.7, 80.2 and 81.6 % of the total oil composition, respectively. while the identified major components in behshahr sample were camphene, 1,8-cineole, chrysanthenone, camphor and borneol, in oil of t. polycephalum plants from baladeh area, 1,8-cineole, chrysanthenone, camphor and borneol, were predominant constituents, and also -pinene, 1,8-cineole, camphor, pinocarvone, iso-menthol, borneol and -eudesmol were major components in sample of savadkooh mountains. amongst them oxygenated monoterpenes were predominant (78.7, 65.2 and 67.6 %, respectively). three samples were differed in 1,8-cineole, chrysanthenone, camphor, borneol, iso-menthol and -pinene. also this study was conducted to investigate in vitro antimicrobial activity of the essential oil of three samples of t. polycephalum against a panel of seven strains of bacteria by disc diffusion method. the zones of growth inhibition measured and the oils were found to have more significant antibacterial activity against gram-positive bacteria (18 to 28 mm) than gram-negative strains (4 to 19 mm). also investigation of minimum inhibitory concentration (mic) and minimal bactericidal concentration (mbc) method, utilizing four microorganism, exhibited the same results. the essential oil of samples showed more significant activity against two gram (+) bacteria (mic/mbc=0.36-1.6/0.72-5.76 g/ml) than gram (-) bacteria (mic/mbc=5.76-12.8/10.24-25.6 g/ml).