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The efficacy of a thermotolerant 6-phytase on performance and apparent total tract digestibility (ATTD) of P and Ca was evaluated in 96 crossbred barrows (8 replicates with 3 pigs each) fed 4 diets. Diets based on corn (Zea mays), a heat-treated grain mix, and soybean (Glycine max) meal with recommended [positive control (PC)] or reduced [negative control (NC)] P and Ca levels were fed from 25 to 115 kg BW. The PC diets contained 0.61% P and 0.72% Ca from 25 to 45 kg, 0.59% P and 0.66% Ca from 45 to 70 kg, and 0.48% P and 0.51% Ca from 70 kg to final BW. The NC diets were reduced in digestible P by 2.0, 2.0, and 1.1 g/kg and in Ca by 1.4, 1.0, and 0.8 g/kg, respectively, for the 3 phases. Phytase was added at 0, 250, and 500 phytase units (FTU)/kg to the NC diet. Performance was measured at the end of each feeding period and ATTD of P and Ca evaluated for a 3-d collection following a 7-d adaptation at an average BW of 45, 70, and 116 kg. Final BW of NC pigs (111.9 kg) was increased (P < 0.05) by P addition (120.3 kg; PC) and by phytase inclusion at both rates (116.5 and 117.6 kg, respectively). The ADG of NC pigs (781 g) was increased (P < 0.05) by both phytase levels (5.5 and 6.6%); pigs fed 500 FTU/kg achieved similar ADG as PC pigs (833 and 858 g, respectively). The G:F was reduced (P < 0.05) in NC pigs (0.308) compared to pigs fed the PC diet (0.328) or diets with 250 or 500 FTU phytase/kg (0.324 and 0.330, respectively). The ATTD of P was lowest in all periods for pigs fed NC diets (35.5% in starter, 32.2% in grower, and 32.1% in finisher period). Phytase increased (P < 0.05) ATTD of P in all periods at both application rates. Pigs fed 500 FTU/kg diet had a higher (P < 0.05) ATTD of P than pigs fed 250 FTU/kg and an ATTD of P at least similar to pigs fed PC. Similar but less pronounced effects were observed for ATTD of Ca. The phytase added to grower-finisher diets with reduced P and Ca nearly restored performance to the level of pigs fed diets with adequate P and Ca.