Abstract: Long-term (14 days) carbon costs of N2 fixation were studied in pot trials. For this purpose the CO2 release from the root space of nodulated and non-nodulated (urea nourished) Vicia faba L. and Pisum sativum L. plants was compared and related to the amount of fixed or assimilated N. Additional measurements of shoot CO2 exchange and dry matter increment were carried out in order to calculate the overall carbon balance. The carbon costs for N2 fixation in Vicia faba 1. (2.87 mg C/mg NfiX) were higher than in Pisum sativum L. (2.03 mg C/mg Nfix). However, the better carbon efficiency in Pisum sativum 1. did not lead to a better growth performance compared to Vicia faba L. Vicia faba L. compensated for the carbon and energy expenditure by more intensive photosynthesis in the N2-fixing treatment. This was not the case with Pisum sativum L., where the carbon balance indicates that the carbon costs of N2 fixation restricted root growth. It is proposed that low carbon costs for N2 fixation indicate an adaptation to a critical carbon supply of roots and nodules, e.g., during the pod-filling of grain legumes.