Dietary phosphorus concentration greatly affects pig’s growth performance, environmental impact and diet cost. A total of 1080pigs (initially 5.9±1.08 kg) from three commercial research rooms were used to determine the effects of increasing standardizedtotal tract digestible (STTD) P concentrations in diets without and with phytase on growth performance and percentage boneash. Pens (10 pigs/pen, 9 pens/treatment) were balanced for equal weights and randomly allotted to 12 treatments. Treatmentswere arranged in two dose titrations (without or with 2000 units of phytase) with six levels of STTD P each. The STTD P levelswere expressed as a percentage of NRC (2012) requirement estimates (% of NRC; 0.45 and 0.40% for phases 1 and 2,respectively) and were: 80%, 90%, 100%, 110%, 125% and 140% of NRC in diets without phytase and 100%, 110%, 125%,140%, 155% and 170% of NRC in diets with phytase. Diets were provided in three phases, with experimental diets fed duringphases 1 (days 0 to 11) and 2 (days 11 to 25), followed by a common diet from days 25 to 46. On day 25, radius samples fromone median-weight gilt per pen were collected for analysis of bone ash. During the treatment period, increasing STTD P from80% to 140% of NRC in diets without phytase improved average daily gain (ADG; quadratic, P<0.01), average daily feed intake(ADFI; quadratic, P<0.05) and gain–feed ratio (G:F; linear, P<0.01). Estimated STTD P requirement in diets without phytasewas 117% and 91% of NRC for maximum ADG according to quadratic polynomial (QP) and broken-line linear (BLL) models,respectively, and was 102%, 119% and>140% of NRC for maximum G : F using BLL, broken-line quadratic and linear models,respectively. When diets contained phytase, increasing STTD P from 100% to 170% of NRC improved ADG (quadratic,P<0.05) and G : F (linear, P<0.01). Estimated STTD P requirement in diets containing phytase was 138% for maximum ADG(QP), and 147% (QP) and 116% (BLL) of NRC for maximum G : F. Increasing STTD P increased (linear, P<0.01) the percentagebone ash regardless of phytase addition. When comparing diets containing the same STTD P levels, phytase increased (P<0.01)ADG, ADFI and G : F. In summary, estimated STTD P requirements varied depending on the response criteria and statisticalmodels and ranged from 91% to>140% of NRC (0.41% to>0.63% of phase 1 diet and 0.36% to>0.56% of phase 2 diet) indiets without phytase, and from 116% to>170% of NRC (0.52% to>0.77% of phase 1 diet and 0.46% to>0.68% of phase2 diet) for diets containing phytase. Phytase exerted an extra-phosphoric effect on promoting pig’s growth and improved theP dose-responses for ADG and G : F.