The relationships between carbon isotope discrimination (?) in mature kernels, canopy temperature depression (CTD) during anthesis and grain filling, 1000-kernel weight (TKW), total carbon content of mature kernels, and yield were studied in durum wheat ( Triticum turgidum L. var. durum ) grown in Spain (western Mediterranean basin). Twenty-five durum wheat genotypes were grown in 2 regions (NE and SE Spain) and under 2 water regimes (rainfed v . support irrigation) from 1997 to 1999 (i.e. a total of 12 trials). Principal component analysis placed yield and ? on the same axis. Pearson’s correlation and stepwise analysis confirmed that ? was the trait that best assessed genotype differences in yield within trials, and was followed, at a considerable distance, by TKW. Our results also demonstrated the extremely poor performance of CTD throughout the wide range of growing conditions in this study.