The extensive grazing systems of northern Australia are dominated by C₄ grasses and are established in N- and P-responsive soils that receive minimal nutrient input. Under these conditions, tropical pasture legumes are expected to improve the quality of grazing forage and fix atmospheric N₂. However, legume persistence is relatively poor, which may be due to a presumed disparity in P requirements among tropical pasture species. This disparity suggests that P-efficient legumes may improve legume persistence, yet the P requirements of many tropical pasture legumes remain unquantified. Nine Desmanthus spp. genotypes were grown in pots to determine differences in shoot yield and root morphology in response to soil P supply (0–100 mg applied P kg^–1 soil; 5–63 mg Colwell P kg^–1 soil). The shoot yield of each genotype increased in response to increased P supply. When P supply was adequate for maximum plant growth, the shoot yield of the best genotype (3.5 g DM pot^–1; JCU 9) was 1.7-fold larger than that of the next most productive genotype. There were also substantial differences in the critical external P requirements of the genotypes (29.4–64.0 mg P kg^–1 soil), although these differences did not always reflect the efficiency of dry matter production per unit of applied P fertiliser. Differences in shoot yield and P acquisition were positively associated with differences in the development of root length. The results indicated that P-efficient genotypes of Desmanthus spp. can be identified for improved growth in the P-responsive pastures of northern Australia. These genotypes may compete more effectively with C₄ grasses and form resilient pasture swards as climate patterns change.