Leaf gas exchange characteristics of a desert annual (Triticum kotschyi [Boiss.] Bowden) and the wheat cultivar TAM W-101 (Triticum aestivum L. em Thell) were compared over a range of leaf water potentials from -0.50 to -2.9 megapascals. At an ambient [CO(2)] of 330 microliters per liter, T. kotschyi had higher conductance and CO(2) assimilation (A) at a given water potential than T. aestivum. Under well watered conditions, A versus internal CO(2) concentration (C(i)) response curves for both species were similar in shape and magnitude, and the higher A of T. kotschyi at an ambient [CO(2)] of 330 microliters per liter was mostly related to the higher stomatal conductance of T. kotschyi. The higher conductance of T. kotschyi than T. aestivum under well watered conditions was associated with higher C(i) and lower water use efficiency. Under water deficits, however, C(i) at 330 microliters per liter ambient [CO(2)] did not differ significantly between species. T. kotschyi had higher A under water deficits than T. aestivum primarily because its A versus C(i) response curves had higher A at C(i) values above about 150 microliters per liter. The results show that conductance played an important role in the high A of T. kotschyi under well watered conditions, but under water deficits the high A of T. kotschyi was related more to the maintenance of a higher capacity for mesophyll photosynthesis.