The role of soil Phosphorus (P) availability on the ecotypic differentiation of plants was investigated. Populations of Aegilops peregrina (Hack.) were collected from four habitats which differed in their soil P. The four soils were: Meron (a P-deficient montmorillonitic xerochrept on dolomite), Malkiya (a P-fertile kaolinitic xerochrept on hard limestone), Har-Hurshan and Bet-Guvrin (lithic xerorthents on soft limestone with appreciable amounts of P, mainly as carbonate-apatite).Plants of the four populations were grown in pots with Meron soil which were previously equilibrated with four different amounts of soluble phosphate to give 1.2, 3.1, 10.7 and 18.9 ΞΌgP g soil of sodium-bicarbonate extractable P. Plants originated from Malkiya population produced higher dry matter yields than the other three populations. Dry matter yields of the various populations were analyzed by means of a Mitcherlich's response function, versus sodium-bicarbonate extractable ('available') soil P. The analysis revealed that Malkiya population plants had a significant advantage over Meron population plants in the response parameter c: this express the response rate of the plants to phosphate by means of dry matter production. With regard to the parameter Po, which represents the ability of plants to utilize barely-available fractions of soil P, the opposite was true. Har-Hurshan and Bet-Guvrin populations were intermediate in these two parameters. A version of the Mitcherlich response function is proposed, which expresses plant yield as a function of both soil 'available' P and plant age.Meron plants contained the highest percentage of P in plant material, but compared to the other populations, they were the most inefficient in producing dry matter from the already absorbed P. Plants from the calcareous soils, Har-Hurshan and Bet-Guvrin, did not show any apparent trend.In soils which contain moderate amounts of lime, P may become a major limiting growth factor. Plant populations became adapted to low availability of P by (1) improving their ability to utilize barely-available soil P fractions and (2), by decreasing their productivity.