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Tepary bean has become popular among poor small-scale farmers in semi-arid Kenya, where it is intercropped with maize. This study aimed at i) evaluating the N-economy of maize/tepary bean intercrop versus sole crop using natural abundance and N-15 enriched fertilizer methods, and ii) assessing the contribution of fixed N-2 by tepary bean to the total N balance in the intercrops and sole cropping systems assessed from harvested seed and residues. Experiments were carried out during the short rains of 2001/2002 and long rains of 2003 at Kenya Agriculture Research Institute (KARI) Kiboko, Kenya. Randomised block design was used with one block devoted to the N-15 natural abundance (-N), the other N-15 labelled fertilizer (+N), replicated 4 times. Above ground biomass and total N were determined in sole crops or intercrops (-N or +N). Tepary bean received 53-69% of its N supply from N-2-fixation with N-2-fixation slightly affected by intercropping or N fertilizer application. N-2-fixation of tepary in greenhouse experiment was lower (36-66%) than in the field study and more affected by N supply. Budgets for N were estimated for field intercrops based on above-ground seed yields, return of crop residues, input of fixed N and fertilizer N. N2-fixation was 59 kg N ha(-1) in plots receiving no N fertilizer, and 73 kg N ha(-1) in plots receiving N as urea. Corresponding fixation by sole tepary was high (87 and 82 kg N ha(-1), respectively), but this advantage was outweighed by greater land use efficiency in intercrop than sole crop.