Phosphorus is a key nutrient element involved in energy transfer for cellular metabolism,respiration and photosynthesis and its supply at low levels can affect legume nodulation,N2 fixation,and C assimilation.A two-year field study was conducted in Ethiopia in 2012 and 2013 to evaluate the effects of P supply on growth,symbiotic N2 nutrition,grain yield and water-use effi-ciency of three groundnut genotypes.Supplying P to the genotypes significantly increased their shoot biomass,symbiotic performance,grain yield,and C accumulation.There was,however,no effect on shoot δ13C values in either year.Compared to the zero-P control,supplying 40 kg?ha-1 P markedly increased shoot biomass by 77％and 66％ in 2012 and 2013,respectively.In both years,groundnut grain yields were much higher at 20 and 30 kg?ha-1 P.Phosphorus supply markedly reduced shoot 815N values and increased the ％Ndfa and amount of N-fixed,indicating the direct involvement of P in promoting N2 fixation in nodulated groundnut.The three genotypes differed significantly in δ15N,％Ndfa,N-fixed,grain yield,C concentration,and δ13C.The phosphorus x genotype interaction was also significant for shoot DM,N content,N-fixed and soil N uptake.