- The composition of hexane extract constituents of ripe mericarps ("achenes") of eleven indigenous populations of Foeniculum vulgare var. vulgare (Apiaceae) was studied. Natural populations were selected along a gradient of annual rainfall from ca. 1000 mm in the northern region down to 125 mm in the Negev desert. Eighteen constituents, with estragole, trans-anethole, fenchone, limonene and alpha-pinene as the major components were separated by GC-MS. Characterized by the level of estragole and trans-anethole, four different groups were obtained: (1) highest estragole (63%) and the lowest trans-ane-thole (3%) characterized the population of Mt. Meron; (2) estra-gole (39-47%) and trans-anethole (17-29%) in 3 mountainous populations; (3) estragole (21-29%) and trans-anethole (38-49%) in the coastal and lowland populations; (4) two exceptional populations with the lowest content of estragole (ca.8%) and high content of trans-anethole (55 and 74%). A reversed association between the content of estragole and trans-anethole suggests a common precursor. In habitats with a high precipitation, the content of estragole was high and that of trans-anethole was low, and vice versa under limited rainfall. It is proposed that the composition of oleoresins of F. vulgare var. vulgare could be governed by environmental conditions. Nevertheless, it is not ruled out that genetic variations account for the recorded differences.