Twenty-five genotypes of durum wheat differing in drought resistance were grown both under rainfed and irrigated conditions in southern Spain during 1999. At the middle of the grain-filling period, chlorophyll fluorescence measurements on flag leaves were recorded in a growth chamber in the laboratory after varying the temperature from 20-60 degrees C at intervals of 5 degrees C. Significant differences were found between genotypes in the sensitivity to high temperatures. The inverse relationship found between the Fv:Fm ratio at 60 degrees C and the grain yield, indicate that this measurement could be of value to identify genotypes of durum wheat which are especially tolerant to heat stress.