The effect of elevated [CO2] (700 μmol/mol) and phosphorus (P) supply on the growth and symbiotic N2 fixation of chickpea (Cicer arietinum L.), field pea (Pisum sativum L.) and barrel medic (Medicago truncatula Gaertn.) were investigated in the glasshouse. The effect of elevated [CO2] on the growth and N2 fixation at various growth stages of the chickpea and field pea plants (grown on a Vertosol) were also examined. Elevated [CO2] generally increased the aboveground biomass of chickpea (by 18–64%), field pea (by 24–57%) and barrel medic (by 49–82%), but the effect was greater when P was non-limiting. Elevated [CO2] only stimulated grain yield of chickpea (by 70%) and field pea (by 21%) if P supply was adequate. Elevated [CO2] reduced the grain protein concentration of chickpea (by 17–18%) regardless of P input, but increased that of field pea (by 11%) when soil P was limiting but had no effect at adequate P. The percentage of shoot N derived from the atmosphere (%Ndfa) of the three legumes was unaffected by elevated [CO2] regardless of soil P supply. Elevated [CO2] increased the amount of N fixed by chickpea (by 20–86%), field pea (by 44–51%) and barrel medic (by 114–250%) under P fertilisation, but had no significant effect when soil P was deficient. These results suggest that the predictions of future climates on the potential contribution of legumes to maintaining soil N fertility will depend on the particular response of a species to soil P status.