The responses of burr medic (Medicago polymorpha) to posphorus (P) was studied on field conditions. The available P-Olsen increased linearly after the application of increasing amounts of P, and approximately 7 kg.ha-1 of P was needed to change the available P in 1 mg.kg-1. The percentage of P in shoots of M. polymorpha increased with increasing P supply, and ranged between 0.18 and 0.25 per cent. Shoot dry weight reached 2682 kg.ha-1 in 1987, and 3050 kg.ha-1 in 1988 both at the highest P application rate. Seed yield ranged between 305 and 681 kg.ha-1 in 1987, and between 300 and 387 kg.ha-1 in 1988. Phosporus utilization efficiency, measured as shoot dry weight per unit of P content, ranged between 391 and 566 kg DW/kg P. Phosphorus use efficiency, measured as shoot dry weight per unit P applied, was 73, 36 and 27 kg DW/kg P, with the application of 24, 62 and 97 kg P.ha-1, respectively. Shoot dry weight increased curvilinearly with increasing P available in the soil at the top 5 cm.