The aim of this study was to determine the chemical variation in indigenous populations of bitter fennel originating from Iran. Essential oils isolated from the fruits of eight different populations of Foeniculum vulgare var. vulgare from different locations of Iran were analyzed by gas chromatography (GC) and GC/mass spectrometry (GC/MS). According to the results, the essential oil content in the fruit varied from 2.7% to 4%. The major chemical constituents exhibited higher differences between the populations. trans-Anethole was found to be the main component of the oil with rates of 46.5% and 84% in fruits and this component was followed by fenchone (9.1â23.8%), which is the second major phenylpropanoid detected in fennel oil. Limonene, estragol (methyl chavicol), γ-terpinene, α-pinene and β-mercene are other main components identified in the fennel oils. trans-Anethole was detected at the highest level (84%) in âZanjanâ population oil and the âMarvdashtâ population contained the highest concentration (12.2%) of estragol compared with other populations. Fenchone, limonene, γ-terpinene, α-pinene and β-mercene were in higher concentrations in âArdabilâ population oil than the other populations, while trans-anethole was the lowest in this populationâs oil. The results suggest that there were important variations among the fennel populationâs fruit in Iran concerning the essential oil content and composition.