The economy of carbon in nodulated white lupin (Lupinus albus L.) was studied in terms of consumption of net photosynthate in nitrogen fixation, in maintenance of respiration, and in the production of dry matter and protein. Net photosynthesis rose to a maximum in early fruiting and then fell abruptly due to shedding of leaves. Nodulated roots acquired translocate equivalent to 51% of the plant's net photosynthate, 78% of the carbon of this translocate being respired, 10% entering dry matter, and 12% returning to the shoot attached to products of nitrogen fixation. Nodules utilized 4·0–6·5 g C in fixing 1 g nitrogen. Photosynthate was utilized most effectively for nitrogen fixation in late vegetative growth. Fruits sequestered 16% of the plant's net photosynthate, shoot night respiration 17%, and dry matter formation in shoot vegetative parts 22%. Averaged over growth, 9·9 g net photosynthate was required to produce 1 g seed dry matter and 31 g net photosynthate to produce 1 g seed protein. Budgets for utilization of the carbon of net photosynthate were constructed for 10 d intervals of the plant's growth cycle. Feeding of shoots with 14CO2 resulted in radiocarbon becoming partitioned approximately as predicted by these budgets. The dependence of root respiration on recent photosynthate was assessed by following the time course of release of 14CO2 to the rooting medium of the 14CO-labelled plants.