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Carbon isotope discrimination (Δ) is a measure of the ratio of the intercellular and atmospheric partial pressures of CO2. Variation in Δ within some C3species, including wheat (Triticum aestivumL.), has recently been correlated with variation in water use efficiency of droughted plants grown in the glasshouse. The objectives of this study were to assess genotypic variation in a larger group of wheat genotypes grown at three locations, and to determine the relationship between yield of field‐grown plants and Δ when water was not a limitation to growth. Twenty‐four bread wheat genotypes, one durum (Triticum turgidumL. durum), and two triticale genotypes (x Triticosecale rimpauiWittm.) were grown in replicated plots at two locations in southern New South Wales, Australia, in 1984. The growing season had above average rainfall, especially in the period preceding antbesis. Carbon isotope discrimination was measured on stem material for each plot. Final aboveground biological yield and grain yield were also measured. There was substantial genotypic variation in Δ. Genotypic ranking for Δ was consistent across field sites and between field‐grown plants and plants grown in pots at another location. A potential source of variation in Δ is variation in stomatal conductance. For pot‐grown plants, the relationship between Δ and total leaf conductance was poor. However, Δ and conductance of the abaxial leaf surface were positively correlated. Under field conditions of nonlimiting water, biological yield and grain yield were both positively correlated with Δ. This latter result indicates that carbon isotope discrimination may be used to select for increased yield in wheat.