Phosphorus deficiency is common in crop production. This pot experiment examined whether the supply of Ca(H2PO4)2 affected acid production by N2-fixing plants of 5 grain and 4 pasture legume species grown in a P-deficient sandy soil for 60-64 days. The legume species differed greatly in their response to P application. Lupinus albus, L. luteus, and Biserrula pelecinus were less responsive to P supply than Cicer arietinum, Vicia faba, Medicago polymorpha, Ornithopus sativus, and Trifolium subterraneum, which in turn were less reponsive than L. angustifolius. Increasing P supply up to moderately deficient levels increased the amount of acid production per unit shoot biomass (specific acid production), and further increasing P decreased specific acid production. The effect of species on acid production was greater than the effect of P supply. Among the species, C. arietinum had the highest specific acid production and B. pelecinus the lowest. Specific acid production was correlated with concentrations of excess cations and ash alkalinity as well as Ca and Mg but not P in shoot. There was a very good agreement between total acid production and total content of excess cations in the whole plant, irrespective of P supply and plant species. The results suggest that acid production by legumes under various P supplies is related to the imbalance of cation and anion uptake.