Technical Abstract: Early planting (late April to early May) is recommended for increasing soybean yield but a full understanding of the physiological response is lacking. This study was conducted to determine whether carbon dioxide exchange rate (CER) could explain this yield difference. A study with five (2007) and six (2009) planting dates was conducted near Ames, IA. Planting dates were between 18 April and 29 May. During the season CER was measured twice weekly. Using published equations, total carbon fixation, dry matter accumulation, and radiation use efficiency were determined for the growth periods R1 to R3, R3 to R6, and R1 to R6. Carbon dioxide exchange rate declined during the early vegetative stages for all planting dates reaching a minimum at approximately the R2 growth stage; followed by a linear rate increase to the late R5 growth stage. During 2007, early planted soybean reached a greater maximum CER, greater R3 to R6 CER, fixed more carbon between R1 and R6, and produced more dry matter by R5.5, but did not increase yield. During 2009 early planting increased dry matter at R5.5 but did not increase yield or other measurements. Yield and seed number were positively related to the R3 to R6 CER. These data demonstrated that early planting did not consistently increase the CER rate during the R3 to R6 growth stage, limiting yield improvement. Factors other than leaf CER are more likely to explain the reported positive yield response to early planting.