Pima cotton (Gossypium barbadense L.) yields have tripled since the mid 1940's due to selection for heat tolerance in advanced breeding programs. This study was conducted to investigate intrinsic differences in photosynthetic capacity between modern and obsolete Pima cultivars under field conditions, and to determine the effects of moderate water stress on ambient gas exchange and photosynthetic capacity of these cultivars. The study was conducted at the University of Arizona's Maricopa Agricultural Center, under irrigated field conditions. Four released cultivars (Pima 32, S-1, S-6, and S-7) were selected for study, representing advancing stages of breeding for heat tolerance and yield over 43 years. These findings indicate that breeding for yield has improved photosynthetic capacity and stomatal conductance in advanced cultivars under conditions of water stress. Our results suggest that further improvements in cotton yield may be achieved through enhanced assimilatory processes in modern cultivars, especially as limits to number of fruit supported per unit of leaf area are approached.