Crop production is a function of leaf area development and photosynthetic rate. Photosynthetic rates can be measured at the canopy level, however the measurements are labor intensive and time consuming. If the relationship between the single leaf and canopy photosynthesis can be established, single leaf photosynthesis can be used as evaluation criteria for crop improvement. Both diurnal and seasonal measurements of canopy photosynthesis simultaneously with single leaf photosynthesis were made as a function of water supply and leaf morphology under field conditions in 1988. Cotton single leaf photosynthetic rates were reasonably well correlated with canopy photosynthetic rates.