A large portion of sows removed from commercial breeding herds in the U.S. are due to lameness. Previous data suggest levels of P fed to developing gilts may be below that required for maximal skeletal development, and that microbial phytase may release less dietary P than expected. The objective of this experiment was to validate the finding that phytase releases less P than previously estimated by dose-response curve experiments using P-deficient diets, and that growing pigs will continue to retain P in the body even after their requirement for P is met. Seventy-two barrows (BW 23.2 ± 1.8 kg) were fed for 30 d. Two basal diets were formulated: a P-adequate diet (AD), meeting all NRC nutrient requirements, including P and Ca, and with an STTD P of 0.36% and a P-deficient diet (DE), meeting all NRC nutrient requirements except P and Ca and with an STTD P of 0.18%; both diets had Ca:STTD P of 2.1. Phytase was added to each of these diets at 0, 350, 600, and 1000 FYT/kg. Phytase improved digestibility of P in both P-adequate and P-deficient diets, but the improvement was smaller in P-adequate diets. Estimates for STTD P release based on our data were 0.07%, 0.09%, and 0.09% for 350, 600, and 1000 phytase units (FYT)/kg in P-deficient diets, and 0.02%, 0.03, and 0.05% in P-adequate diets. In P-deficient diets, phytase improved absorption and retention of P, and increased urinary excretion of P. In P-adequate diets, phytase improved absorption of P, tended to improve retention of P, and increased urinary excretion of P. Phytase improved digestibility (ATTD) of Ca in P-deficient diets, but not in P-adequate diets. There was a slight improvement in ATTD of dry matter when 350 or 600 FYT phytase/kg was used, and a tendency for phytase to improve ATTD of gross energy and crude protein, regardless of whether the diet was adequate or deficient in P. In conclusion, phytase was effective in improving utilization of P, but these data suggest phytase may not be as effective when added to diets that are not deficient in P. The data indicate that pigs can retain more P than what is required for growth, but do not fully confirm our previous findings since absorption of P did not increase as substantially as in the previous experiment. These data confirm that diets provided to developing gilts should be formulated to exceed the established requirement for STTD P, especially when phytase is being used to improve the availability of dietary P. These data also beg the question about assumed P release by phytase in diets for mature sows.