abstract the antibacterial effects of essential oils isolated from zataria multiflora boiss and mentha longifolia (l.) hiuls. on escherichia coli and staphylococcus aureus were studied. the disc diffusion method was used to evaluate the zone of microbial growth inhibition at various concentrations of the oils. the antibacterial effect was also studied against three different concentrations of microbial suspension to find out the minimal inhibitory concentration (mic) and minimal bactericidal concentration (mbc). the oils from the above plants were found to be strongly bactericidal with that of zataria multiflora being more effective. the oil from z. multiflora and m. longifolia were analyzed by gc and gc/m s and 22 and 21 components were identified, respectively. the profile of the oil components from both plants was similar in that -caryophyllene, -pinene, limonene, myrcene and sabinene were found at different concentrations. the major components of z. multiflora oil were carvacrol (37%), p-cymene (15%) and dodecane (9%), and those of m. longifolia were piperitone (43.9%), limonene (13.7%) and trans-piperitol (12.9%). the high carvacrol content of z. multiflora oil accounts for its strong antimicrobial activity. the oil of m. longifolia contained high levels of limonene which seems to have imparted bactericidal property to the oil. key word index: zataria multiflora mentha longifolia lamiaceaeessential oil compositionp-cymenecarvacrollimonene trans-piperitolpiperitoneantimicrobial activity