abstract the composition of the essential oils from the aerial parts of three labiatae species of iran: teucrium persicum boiss., thymus caucasicus willd. ex ronniger subsp. grossheimii (ronniger) jalas. and marrubium crassidens boiss. obtained by hydrodistillation were analyzed by gc and gc/ms. epi--cadinol (23.2%) and -pinene (17.3%) were the main components among the thirty-one constituents characterized in the oil of teucrium persicum representing 95.9% of the total components detected. seventeen compounds were identified in the oil of thymus caucasicus representing 99.6% of the total oil with thymol (34.2%), methyl chavicol (25.1%) and -terpinene (12.7%) as the major constituents. the oil of marrubium crassidens was characterized by higher amount of -caryophyllene (20.3%), germacrene d (12.9%), caryophyllene oxide (11.1%) and cubenol (11.0%) among the fourteen compounds comprising 91.7% of the total oil detected. the oils (teucrium persicum and marrubium crassidens) consisted mainly of sesquiterpenes, while in thymus caucasicus oil monoterpenes predominated over sesquiterpenes. key word index: teucrium persicum thymus caucasicus marrubium crassidens lamiaceaeessential oil compositionepi--cadinolthymolmethyl chavicol-terpinene-caryophyllenegermacrene dcaryophyllene oxidecubenol