The apparent photosynthesis (AP) rate of three varieties of soybeans (Glycine max (L.) Merrill) (‘Wayne,’ ‘Harosoy,’ and a nearly isogenic ‘Harosoy’ line with narrow leaflets) with different leaf and canopy characteristics was measured with a closed canopy system in the field for 23 days between mid‐July and mid‐August 1967 at Urbana, Ill. Solar radiation was measured with an Eppley pyrheliometer whenever AP measurements were made. The closed canopy system also provided an estimate of the transpiration rate.Increasing the CO2 concentration from 300 to 600 ppm resulted in an average increase, across varieties, of 72% in the mean daily AP rate. Mean daily AP rate of Harosoy normal dropped 87% as the CO2 level decreased from 300 to 100 ppm. Increasing the CO2 concentration from 300 to 600 ppm resulted in a decrease in transpiration for all three varieties.When AP rates at 300 ppm were expressed on a ground area basis, Wayne fixed 6.3% more CO2 than Harosoy narrow leaf. However, when AP was expressed on a leaf area basis, Harosoy narrow leaf fixed 45% more CO2 than Wayne and Harosoy normal fixed 13% more than Wayne.At 300 ppm CO2 none of the varieties reached light saturation at radiation intensities commonly encountered in the field. The average efficiency of the three varieties at 300 ppm CO2 was 2.16%. Under the conditions encountered in this study, apparently both the supply of CO2 to the reaction site in the leaf and the radiant energy available to fix CO2 were limiting AP.