Soybean Glycine max (L.) Merr. genotypes express different rates of photosynthesis per unit leaf area as measured on single leaflets of similar ontogeny in similar environments. Knowledge of the factors that cause genotypic variation in photosynthetic rate would be useful;In 1982, two genotypes bred for rapid and two for slow leaf photosynthesis were grown in the field. Near the beginning of rapid seed growth, CO(,2) and H(,2)O exchange characteristics of recently expanded terminal leaflets were measured at two leaf temperatures, 28 and 18 C. Leaf and cell dimensions also were measured. Differences in apparent photosynthesis per unit leaf area (AP) were not attributable to differences in stomatal resistance, in mesophyll cell size or number, or to the efficiency of the photochemical reactions. Factors internal to the cell were suggested to be causative;In 1983, six cultivars adapted to Iowa were grown in the field. Total photosynthesis (TPs), estimated by ('14)CO(,2) uptake, number of chloroplasts, amounts of soluble protein and chlorophyll, and activities of ribulose 1,5-bisphosphate carboxylase (EC 4.1.1.39) and uncoupled photosynthetic electron transport, all on a unit leaf area basis, were determined for leaves of similar age and nodal position on six dates after full leaf expansion. Among cultivars, TPs was significantly correlated with carboxylase activity, and with protein and chlorophyll contents. On later dates, TPs also was correlated with electron transport. The data suggest, but not unequivocally, that cultivars with more rapid TPs had more chloroplasts beneath a unit of leaf surface;The data also showed that (1) reducing leaf temperature from 28 to 18 C caused a 90% increase in residual resistance and a 18% decline in AP; (2) differences in TPs among leaves within a cultivar were associated with differences in carboxylase activity, protein and chlorophyll contents and ultimately in chloroplast quality rather than number; and (3) during senescence, the decline in TPs was associated with a decline in chloroplast quality rather than number.