Carbon-isotope discrimination (Delta) is used to distinguish between different photosynthetic pathways. It has also been shown that variation in Delta occurs among varieties of C(3) species, but not as yet, in C(4) species. We now report that Delta also varies among genotypes of sorghum (Sorghum bicolor Moench), a C(4) species. The discrimination in leaves of field-grown plants of 12 diverse genotypes of sorghum was measured and compared with their grain yields. Discrimination varied significantly among genotypes, and there was a significant negative correlation between grain yield and Delta. The variation in Delta may be caused by genetic differences in either leakiness of the bundle-sheath cells or by differences in the ratio of assimilation rate to stomatal conductance. At the leaf level, the former should be related to light-use efficiency of carbon fixation and the latter should be related to transpiration efficiency. Both could relate to the yield of the crop.