The effects of phosphate deficiency on the composition and photosynthetic CO2 assimilation rates of fully expanded leaves of sunflower, maize and wheat plants are described. The regulation of photosynthesis by stomatal and mesophyll characteristics of leaves of different phosphate status is analysed and related to structure. Phosphate deficient leaves had small concentrations of inorganic phosphate, Pi, in the tissue water. Rate of photosynthesis in leaves and stomatal conductance were smaller in plants grown with inadequate phosphate when measured under any given light intensity or CO2 partial pressure. Despite the decrease in stomatal conductance (and without evidence of patchy stomatal closure), the relative stomatal limitation of photosynthesis was similar in the plants grown with deficient or abundant phosphate. However, the mesophyll capacity for photosynthesis was greatly limited by phosphate deficiency. Leaves deficient in phosphate had larger numbers of small size cells per unit leaf area than leaves with adequate phosphate. The total soluble protein content of leaves decreased with phosphate deficiency in all three species; however, the leaf chlorophyll content was decreased only in sunflower and maize and not in wheat. These results suggest that stomatal conductance did not restrict the CO2 diffusion rate, rather the metabolism of the mesophyll was the limiting factor. This is shown by poor carboxylation efficiency and decreased apparent quantum yield for CO2 assimilation, both of which contributed to the increase in relative mesophyll limitation of photosynthesis in phosphate deficient plants.