The quantum yields of C(3) and C(4) plants from a number of genera and families as well as from ecologically diverse habitats were measured in normal air of 21% O(2) and in 2% O(2). At 30 C, the quantum yields of C(3) plants averaged 0.0524 +/- 0.0014 mol CO(2)/absorbed einstein and 0.0733 +/- 0.0008 mol CO(2)/absorbed einstein under 21 and 2% O(2). At 30 C, the quantum yields of C(4) plants averaged 0.0534 +/- 0.0009 mol CO(2)/absorbed einstein and 0.0538 +/- 0.0011 mol CO(2)/absorbed einstein under 21 and 2% O(2). At 21% O(2), the quantum yield of a C(3) plant is shown to be strongly dependent on both the intercellular CO(2) concentration and leaf temperature. The quantum yield of a C(4) plant, which is independent of the intercellular CO(2) concentration, is shown to be independent of leaf temperature over the ranges measured. The changes in the quantum yields of C(3) plants are due to changes in the O(2) inhibition. The evolutionary significance of the CO(2) dependence of the quantum yield in C(3) plants and the ecological significance of the temperature effects on the quantum yields of C(3) and C(4) plants are discussed.