Tropical pasture legumes such as Desmanthus are expected to improve the productivity of the extensive grazing systems of northern Australia. This is because legumes increase forage quality and fix atmospheric nitrogen. However, little is known about the influence of soil phosphorus (P) supply on legume growth and nodulation in mixed pasture swards that include highly-competitive C4 grasses. Furthermore, predicted increases in CO2 concentration may influence the relative competitiveness of tropical pasture legumes. A controlled-environment pot trial was conducted to investigate the influence of CO2 concentration (250, 500 and 750 ppm) and soil P supply (3, 10 and 40 mg P kg–1) on the growth of two Desmanthus spp. cultivars (cvv. JCU 7 and JCU 9). The legumes were grown both as monocultures and as mixed swards with Premier Digit (Digitaria eriantha). Legume shoot yields and tissue P concentrations increased in response to soil P supply. On average across the treatments, the proportion of cv. JCU 9 in the mixed grass/legume swards was 1.9-fold higher than that of cv. JCU 7. In general, legume competitiveness decreased at higher CO2 concentrations. Under monoculture conditions, legume nodule size increased in response to soil P supply but not CO2 concentration, and the nodules of cv. JCU 9 were generally larger than that of cv. JCU 7. These results demonstrate that there are differences in competitiveness and nodulation among cultivars of Desmanthus which are likely to influence forage quality and atmospheric nitrogen-fixation. Consideration should be given to both shoot yield and potential nitrogen-fixation when applying P fertiliser for optimum pasture production.