The objectives of these experiments was to evaluate super-dosing phytase as a way to enhance growth performance of growing pigs and to understand the mechanism behind the performance improvement. One experiment used 440 pigs (6.27 ± 0.01 kg) in a 2 × 2 factorial treatment design comparing the main effects of diet (NRC derived positive control [PC] versus a negative control: 10% lower SID lysine and 1% reduced fat [NC]) and phytase levels (0 vs 2,500 FTU phytase). Phytase improved ADG and G:F (P < 0.05). Experiment 2 was conducted on 2,200 growing pigs (36.6 ± 0.3) allotted to 5 treatments: a balanced PC (250 FTU phytase/kg), a NC (15% less SID lysine and 1.5% lower NE), and 3 SD treatments applied to the NC for a total of 1,000, 1,750, and 2,500 FTU phytase/kg. Phytase improved carcass feed and energy efficiency (P < 0.05). The 3 remaining experiments used 32 gilts fitted with t-cannulae at the terminal ileum (39.7 ± 0.3 kg, 60.5 ± 0.5 kg, and 82.5 ± 0.7 kg, for Exp. 3, 4 and 5, respectively) allotted to 4 treatments: a corn-soy control diet containing 250 FTU phytase/kg, and 3 SD treatments with phytase added to 1,000, 1,750, and 2,500 FTU phytase/kg, respectively. Chromic oxide was added at 0.4% as a marker. In Exp. 3, SD increased the apparent ileal digestibility (AID) of ether extract and dry matter (DM), decreased the apparent total tract digestibility (ATTD) of phosphorus and decreased calcium excretion (P < 0.05). In Exp. 4, SD increased the ATTD of nitrogen and DM, and decreased calcium excretion (P < 0.05). In Exp. 5, SD increased the AID of DM, GE, and starch, and decreased the ATTD of phosphorus and calcium, and decreased calcium excretion (P < 0.05). Super-dosing phytase decreased the concentration of phytate and inositol-5-phosphate, and increased the concentration of lower inositol derivatives and myo-inositol in Exp. 3, 4, and 5 (P < 0.05). Therefore, the growth improvements observed with SD are unlikely a result of improved nutrient or energy digestibility, but may be related to phytate degradation and increased myo-inositol availability.