1. Three experiments were conducted to evaluate the effects of the dietary addition of fungal phytase (derived from Aspergillus niger) on the performance and phosphorus utilisation in broiler chickens receiving low phosphorus diets without additional inorganic phosphates. 2. Graded amounts of supplemental phytase (125, 250, or 500 PU/kg diet) resulted in significant increases in both growth rate and food intake. However, only moderate improvements in food conversion were noted. 3. The enhancement of chick performance was related to an improved utilisation of dietary phosphorus, as confirmed by significantly elevated plasma concentrations of inorganic phosphorus and increased tibia ash percentages in birds receiving phytase-treated diets. The apparent availability of phosphorus was markedly improved and its concentration in excreta was reduced (experiment 1, P < 0.05). 4. It was concluded that an inclusion of phytase into practical broiler diet will allow the reduction or omission of additional dietary inorganic phosphorus.