Gas exchange measurements and leaf anatomy of 10 cassava cultivars were conducted to study the interrelationship between the relatively high photosynthetic rates and the factors limiting internal CO2 diffusion. The internal mesophyll surface area per unit leaf surface area (A(mes)/A) and the intracellular components of CO2 diffusion and fixation resistance (R(cell)CO2) were determined. Among the group of cultivars tested net CO2 exchange rates were 26±2.5 μmol CO2 m(-2) s(-1) in normal air and intense light and A(mes)/A ranged from 14 to 38. Estimated R(cell)CO2 ranged from 4300 to 13,000 s m(-1). The combined and compensating effects of A(mes)/A and R(cell)CO2 accounted for both the high net photosynthetic rates (Pn) and the lack of large differences in Pn among cultivars.