The photosynthetic and respiratory net CO2 exchange potential of the fruit of Pisum sativum was evaluated in one normal and three mutant genotypes differing widely with respect to foliage area and morphology: AfAf.StSt, conventional; AfAf.StSt, vestigial stipules; afaf.St.St, all leaflets replaced by tendrils but stipules normal (here termed ‘semi-leafless’) and afaf.stst, tendrils and vestigial stipules (‘leafless’). Agronomically the latter phenotype offered improvement in resistance to lodging, crop drying, and harvester through-put. On a dry weight basis, fruits of the leafless mutant were consistently more active photosynthetically in terms of CO2 uptake from the atmosphere during the initial 18 days post-anthesis than were the corresponding fruits of the other three phenotypes. During the subsequent 16-day period of seed filling there was no marked difference between phenotypes and the fruit continuously lost CO2 to the atmosphere, but significantly less CO2 was lost in the light (40 k lux) than in the dark. That the fruit of the leafless mutant may therefore benefit from the increase in light available to the fruit within a sward canopy comprised of tendrils in place of leaflets is discussed. However, there was still a significant reduction in seed yield associated with the leafless mutant despite a normal complement of ovule initials. The growth curves and mean dry weights per seed were not significantly different between phenotypes. In fruit of the conventional phenotype the attainment of maximal pod wall weight coincided with the initiation of seed fillingonday 15 after anthesis. In a series of isogenic lines the gene af delayed maximal wall development by 6 days whilst the gene st markedly lowered the maximum wall weight attainable. Possible causes of yield reduction in the leafless mutant are discussed in relation to the observed action of these genes in the homozygous double recessive condition.