Growth and N utilization responses of pigs to Aspergillus niger phytase supplementation of low-protein or high-phytin diets were examined in 2 experiments. In the first experiment, 6 diets arranged in a 3 x 2 factorial with Aspergillus niger phytase at 0, 600, or 1,200 units/kg and dietary protein at 160 or 200 g/kg; and 12 pigs (growth) or 6 pigs (N utilization) per diet were used to test the hypothesis that phytase ameliorates dietary protein effects on growth and nutrient balance. In the second experiment, 4 diets arranged in a 2 x 2 factorial of phytase at 0 or 1,200 units/kg and phytin P at 2.2 g/kg (3.2 g/kg of total P) or 3.9 g/kg (4.9 g/kg of total P); and 12 pigs (growth) or 8 pigs fitted with ileal T-cannulas in a replicated 4 x 4 Latin square (for ileal digestibility of AA) per diet were used to test the hypothesis that increased phytin depresses nutrient utilization and phytase addition ameliorates these effects. Whereas reduced dietary protein depressed (P < 0.001) BW gain and N utilization, phytase in low-protein diet did not alleviate these effects. Although phytase ameliorated phytin-induced reduction in ileal P digestibility (P < 0.001), this was not accompanied by any change in ileal AA digestibility. Given that phytase improved (P < 0.001) P, but not N utilization of diets fed in both studies, phytase-induced improvement in BW gain of pigs fed low-phytin P diets is independent of changes in N utilization.