Carbon isotope discrimination (ฮ) has been suggested as a selection criterion to improve transpiration efficiency (W) in bread wheat (Triticum aestivum L.). Cultivars 'Chinese Spring' with low A (high W) and 'Yecora Rojo' with high ฮ (low W) were crossed to develop F1, F2, BC1, and BC2 populations for genetic analysis of ฮ and other agronomic characters under well-watered (wet) and water-stressed (dry) field conditions. Significant variation was observed among the generations for ฮ only under the wet environment. Generation x irrigation interactions were not significant for ฮ. Generation means analysis indicated that additive gene action is of primary importance in the expression of ฮ under nonstress conditions. Dominance gene action was also detected for ฮ, and the direction of dominance was toward higher values of ฮ. The broad-sense and the narrow-sense heritabilities for ฮ were 61 % and 57% under the wet conditions, but were 48% and 12% under the draughted conditions, respectively. The narrow-sense heritabilities for grain yield, above-ground dry matter, and harvest index were 36%, 39%, and 60% under the wet conditions and 21%, 44%, and 20% under dry conditions, respectively. The significant additive genetic variation and moderate estimate of the narrow-sense heritability observed for ฮ indicated that selection under wet environments should be effective in changing ฮ in spring bread wheat.