Rates of net photosynthesis of the flag leaves of 15 genotypes of wheat and related species were measured throughout their life, using intact leaves on plants grown in the field. At the stage when rates were maximal, they were in general highest for the diploid species, intermediate for the tetraploid species and lowest for T. aestivum (means of 38, 32 and 28 mg CO2 dm-2 h-1, respectively). Rates were strongly negatively correlated with leaf area, leaf width and the mean plan area per mesophyll cell and positively correlated with stomatal frequency and number of veins per mm of leaf width. The differences among species in these attributes were mainly related to ploidy level. It was not possible to determine the relative importance of each anatomical feature, although the changes in stomatal frequency had only slight effects on stomatal conductance; the observed differences in rates of photosynthesis were much greater than would be exposed from those in stomatal conductance alone. There was genetic variation in rates of light dependent O2 evolution of isolated protoplasts and intact chloroplasts but no difference attributable to ploidy. The mean rate, 91 .mu.mol O2 mg-1 chlorophyll h-1, equivalent to 3.9 mg CO2 mg-1 chl h-1 was considerably less than the rate of photosynthesis in comparable intact leaves, which was 7.2 chl h-1. The total above-ground dry matter yields were least for the wild diploids T. urartu and T. thauodar and the wild tetraploid T. dicoccoides, but the other wild diploids produced as much dry matter as the hexaploids. The prospects of exploiting differences in photosynthetic rate in the breeding of higher yielding varieties are discussed.