A trial was conducted to evaluate the dose response of a novel microbial 6-phytase expressed in Aspergillus oryzae (Ronozyme HiPhos; DSM Nutritional Products, Basel, Switzerland) in pigs. Forty-eight individually housed pigs (Landrace × Pietrain; 52 kg BW; 24 males and 24 females) were distributed among 6 experimental treatments consisting of a low-P diet (3.5 g P/kg; 1.1 g digestible P/kg), which was supplemented with 500, 1000, 2000, or 4000 units of phytase activity/kg, and a standard-P diet (4.5 g P/kg; 1.8 g digestible P/kg) that was supplemented with CaHPO(4). After 17 d, fresh feces were sampled from all pigs and the apparent total tract digestibility of DM, OM, ash, P, and Ca was measured using TiO(2) as indigestible marker. Blood samples were also obtained from each pig and serum was analyzed for P and Ca concentrations. The nonsupplemented low-P diet increased Ca and reduced P blood serum concentrations (P < 0.05) relative to the standard-P diet (10.8 vs. 10.2 and 6.7 vs. 7.7 mg/dL, respectively). Phytase supplementation of the low-P diet reduced Ca (from 10.8 to 9.9 mg/dL; linear, P < 0.001) and increased P concentrations (from 6.7 to 8.0 mg/dL; linear and quadratic, P < 0.001) in serum and reduced P concentration in feces (from 13.7 to 7.6 g/kg DM; linear and quadratic, P < 0.001). Phytase improved the total tract digestibility of P (from 29.0 to 62.3%; linear and quadratic, P < 0.001 and P < 0.05), Ca (from 54.0 to 75.7%; quadratic, P < 0.01), and ash (from 46.2 to 57.7%; quadratic, P < 0.01). In conclusion, the microbial 6-phytase tested improves the apparent total tract digestibility of P in growing pigs and reduces P excretion in feces in a dose-dependent manner.