abstractthe alarming resistance of many bacterial pathogens to most available antibiotics has increased interest in natural plant products as alternative antibacterial agents. in this research, the antibacterial activity of teucrium polium essential oil was determined against urinary isolates of klebsiella pneumoniae. the aerial parts of t. polium were collected at full flowering stage in tehran, iran. the essential oil was isolated by hydrodistillation and analyzed by a combination of capillary gas chromatography (gc) and gas chromatography-mass spectometry (gc-ms) antibacterial activity was measured against fifteen clinical isolates of k. pneumoniae by disc diffusion. minimum inhibitory and bactericidal concentrations were also determined using broth microdilution. twenty constituents were detected in t. polium essential oil of which the major components were -caryophyllene (29%), farnesene (13%), -pinene (11%) followed by germacrene d (6.5%) and -pinene (5.5%). all k. pneumoniae clinical test isolates were susceptible to the essential oil by disc diffusion with inhibition zones within the range of 14-28.5 mm. minimum inhibitory and bactericidal concentration values were 0.62-1.25 mg/ml and confirmed the disc test results. t. polium essential oil may have a potential for use against multidrug resistant organisms such as clinical isolates of k. pneumoniae.keywords: essential oilteucrium poliumantibacterial activityklebsiella pneumoniae acknowledgementsthe authors wish to thank shahid beheshti university research council for providing a special grant to finance this research.