Pachyrhizus ahipa and Pachyrhizus erosus are tuber legume crops with high yield potential, high nutritional value, low N fertiliser and low pesticide requirement and have a potential use in worldwide cultivation. One of the main advantages of these two crop species are their ability to produce high biomass without N fertiliser application. In some climatic regions P. erosus is reproductively pruned in order to obtain economic yields, but little is known about how the pruning influences the capacity of these tuber legumes to fix nitrogen. Two experiments were carried out to investigate the effect of flower pruning on the yield of tuberous roots, the capacity of N-2 fixation and net N balance. In the first experiment two landraces of P. ahipa and three cultivars of P. erosus were grown with or without flower pruning. The difference method was used to estimate N-2 fixation using maize and sorghum as reference crops. In the second experiment N-15 isotopic dilution methodology was used to determine N-2 fixation in the same cultivars as in Experiment 1, using the same reference crops, but tuber legumes were only grown with pruning of flowers. In the first experiment the amounts of nitrogen fixed ranged from 58 to 80 kg N ha(-1) for P. ahipa and from 163 to 216 kg N ha(-1) for P. erosus, and this variable was not significantly affected by flower pruning treatments, even-though treatments with flower pruning were harvested earlier. With pruning of flowers the amount of N in the residue ranged from 55 to 151 kg N ha(-1) and their N concentration ranged from 32 to 35 g kg(-1). In the second experiment the % N derived from the atmosphere (Ndfa) was estimated to range from 55 to 69% for P. ahipa and from 68 to 77% for P. erosus, while the amount of N fixed ranged from 74 to 95 kg N ha(-1) and from 172 to 190 kg N ha(-1) respectively, for these species. Positive net N balances in the system ranged from 12 to 18 kg N ha(-1) for P. ahipa and from 55 to 81 kg N ha(-1) for P. erosus. (C) 1997 Elsevier Science Ltd.