Genetic variation in photosynthetic rate, stomatal conductance, leaf chlorophyll content, and dark respiration was studied in 16 wheat genotypes (Triticum aestivum L.) grown under high-temperature field conditions in central Mexico. Measurements were made on flag leaves in full sun light, at three different stages of plant development, and at different times of the day. Two experiments were conducted with sowing dates in December 1991 and March 1992, and the results compared with the grain yields of the experimental plots on which measurements were made. Photosynthetic rate was fairly stable during the day between 10:00 and 14:00, and across experiments, despite differences in leaf temperature of up to 4°C. However, rates of photosynthesis fell noticeably at successively later stages of plant development, and showed a strong interaction with genotype. For both planting dates, there was a clear association among the parameters photosynthetic rate, stomatal conductance, leaf temperature, and chlorophyll content, and all of these parameters were correlated significantly with grain yield. Photosynthetic rate in the post-anthesis period was strongly associated with chlorophyll loss after anthesis, which suggests that genetic variability in the onset of senescence may be an important variable determining the yield of wheat in hot environments.