Technical Abstract: Some reports indicate that mesophyll conductance to carbon dioxide varies greatly with the sub-stomatal carbon dioxide concentration during the measurement, while other reports indicate little or no change. I used the oxygen sensitivity of photosynthesis to determine the response of mesophyll conductance to sub-stomatal carbon dioxide concentration over the range of about 100 to 300 ppm sub-stomatal carbon dioxide concentration at constant temperature in common bean and soybean grown over a range of temperatures and light levels. In soybean grown and measured at high light there was only a slight decrease in mesophyll conductance with sub-stomatal carbon dioxide concentration. With lower light during the measurement of mesophyll conductance, and especially with low light during plant growth, there was a substantial decrease in mesophyll conductance with sub-stomatal carbon dioxide concentration. In common bean, the same range in sub-stomatal carbon dioxide concentrations resulted in much larger decreases in mesophyll conductance than in soybean. Growth temperatures of 20 to 30 C had little influence on the response of mesophyll conductance to sub-stomatal carbon dioxide concentration or its absolute value in either species. It is concluded that these two species differed substantially in the sensitivity of mesophyll conductance to sub-stomatal carbon dioxide concentration, and that light but not temperature during leaf development strongly affected the response of mesophyll conductance to sub-stomatal carbon dioxide concentration.