Narrow-leafed lupin (Lupinus angustifolius L.) is usually grown in sandy, acidic and phosphorus (P) deficient soil with low yield and variable harvest index. This study aimed to examine the effects of varying P supply on lupin growth, seed yield and harvest index. Non-abscission plants (cv. Danja) were grown in Lancelin sand at seven rates of P supply (5, 10, 15 20, 25, 30 or 40 mg kg(-1)) in a naturally-lit glasshouse. The rate of leaf emergence, flowering time and flower number were decreased or delayed by low P supply (5, 10 or 15 mg kg(-1)) with no differences at P rates higher than 20 mg kg(-1). High P supply (25. 30 or 40 mg kg(-1)) increased plant seed yield and harvest index largely by increasing the number of pods and consequently yield on the lateral branches, but had less effect on the number of seeds per pod and seed size. Seed yield and seed P concentration continued to increase up to 40 mg P kg(-1) but harvest index plateaued at 25 mg P kg(-1), indicating that low P supply decreased reproductive growth more than vegetative growth in narrow-leafed lupin.