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Chemical analysis of the volatile fraction of oleoresins from fruits of seven natural populations of Foeniculum vulgare var. vulgare (bitter fennel), from the wild and after cultivation indicated the presence of two groups of populations. The first included three indigenous populations with a profile of constituents similar to that observed in their cultivated representatives, suggesting a high chemical heritability. In the second group, with four populations, the major chemical constituents exhibited higher differences between the natural populations and their cultivated representatives, suggesting a lower heritability. Cluster analysis within the first group revealed a remarkable similarity between the chemical composition of the oleoresins from plants in nature, and that of their cultivated representatives, suggesting two different chemotypes: a trans-anethole chemotype, represented by the populations of the Negev desert and of the northern coastal plane, and an estragole dominated chemotype of a northeastern population of Mt. Dov. It is conjectured that northwestern winds, during the flowering season (July-October) restrict free westwards movement of pollinators. As a result, panmictic pollination is adversely affected, enhancing isolation and genetic differentiation. In populations of lower heritability, chemical response to cultivation was variable. In two mountainous populations (Mt. Tayyasim, and Mt. Meron) cultivation had reduced trans-anethole, whereas in two other populations of lower elevations (Ramat-ha’sharon and Ma’alot-Tarshicha), cultivation bad decreased the relative content of estragole and elevated that of trans-anethole. Either chemotypic differentiation or phenotypic plasticity increases within species chemical variability, but the specific ecological roles of these essential oils remain to be uncovered. (C) 2002 Elsevier Science Ltd. All rights reserved.