Grain legumes rely on N-fixation to provide N when soil-available mineral N is insufficient.  For legume crop growth models, it is important to accurately simulate the dynamics of nodulation and N-fixation relative to insufficient N uptake.  In this paper we describe the processes and relationships of N-fixation to plant N availability in the CROPGRO-legume model.  Nodule growth rate and N-fixation are simulated on a land area basis.  There is a small initial nodule mass per plant at emergence, a maximum relative growth rate (RGR), and a maximum specific nitrogenase activity (SNA), with the latter two being sensitive to soil temperature and plant water status. Assimilate is used for nodule mass growth based on tissue composition and 7.06 g glucose is required for per g N fixed.  There is a minimum “by-pass” flow of the root assimilate that must go to nodules.  If today’s N uptake is insufficient for growing plant tissue at the target N concentration, assimilate is allocated to nodules to fix that amount of N deficit in the following order of priority:  1) N fixation to limit of SNA, and 2) if nodule mass times SNA is too low, then assimilate is used for nodule growth, as limited by nodule mass times RGR.  If N-fixation is not sufficient to supply today’s plant N demand, carbohydrates accumulate in the shoot.  Dynamics of nodule growth and N-fixation show a minor dip in leaf N concentration and minor increase in shoot carbohydrate, even while nodule mass may be rapidly increasing.  For crops like soybean and peanut, such symptoms are relatively rare under field conditions, so we set constants, initial mass, by-pass flow, and temperature sensitivities of SNA and RGR to mimic this assumed field behavior.  The model has performed well in limited tests against time-series data on nodule mass and N-fixation.