Phosphorus is the limiting nutrient for legume symbiotic nitrogen fixation in most soils of the planet. Thus the yield of legumes generally responds to phosphorus fertilization. The P requirement of legumes is generally higher when their growth depends upon atmospheric nitrogen than upon ammonium or nitrate. This communication will revise the metabolic and structural utilizations of P that may limit the P efficiency for N2 reduction in legume nodules and their genetic variation among legume species. The interaction with P metabolism will be illustrated with our results on nodule phytase activity (Araujo et al. 2008), and the tissue expression of the corresponding gene by in situ RT-PCR on nodule sections. The later will be compared with that of phosphoenol pyruvate phosphatase, and other candidate genes like trehalose 6-P phosphatase, that have been identified by deep super SAGE on nodule mRNA populations (Molina et al. 2011). The selection procedure of contrasting genotypes in P utilization for N2 fixation will be proposed to assess the functions involved in the adaptation of N2-fixing legumes in agro-ecosystems with low P soils, and to improve the yield of legumes and the sustainability of agricultural systemsdepending upon N2-fixation.