Two experiments were conducted to evaluate the effect of dietary phytase and increasing levels of available phosphorus (P(av)) on the growth performance and phosphorus metabolism of broiler chicks. In both experiments, graded levels of P provided by dicalcium phosphate and of phytase were added to a low-P corn-soybean meal basal diet. In Experiment 1, diets providing .21, .29, .37, and .44% P(av) without phytase; .21% P(av) plus .05, .10, or .30% phytase; and .29% P(av) plus .10% phytase were each fed to four groups of seven chicks, 3 days of age. In Experiment 2, diets providing P(av) levels of .32, .38, and .44% and phytase levels of .5, 1.0, and 1.5% (250, 500, and 750 units/kg) in a factorial arrangement were each fed to four groups of eight chicks, 5 days of age. In Experiment 1, increasing dietary P(av), but not phytase, increased feed intake, weight gain, feed conversion, plasma inorganic P, tibia and toe ash, and tibia breaking strength (P < or = .05). Plasma inorganic P responded quadratically to increasing dietary phytase. In Experiment 2, feed intake and weight gain were increased by elevating the level of P(av), but not by phytase. Toe and tibia ash and plasma inorganic P were increased by dietary phytase and increasing levels of P(av) (P < or = .01). Tibia breaking strength was improved (P < or = .05) by dietary phytase but not by increasing levels of P(av). The P excretion was elevated (P < or = .01) by increasing levels of P(av) and was decreased by supplemental phytase (P < or = .05).(ABSTRACT TRUNCATED AT 250 WORDS)