Pigs digest P in plant feedstuffs poorly because pigs do not produce sufficient endogenous phytase to hydrolyze P from phytate (inositol hexaphosphate). Supplementation of phytase to diets of piglets and grower-finisher pigs increased digestibility of minerals including P and Ca; however, data on phytase efficacy in lactating sows are scarce. Therefore, effects of adding a bacterial 6-phytase expressed in a strain of Aspergillus oryzae (Ronozyme HiPhos; DSM Nutritional Products, Basel, Switzerland) on apparent total tract digestibility (ATTD) of P and Ca was assessed in 45 lactating sows. Three diets were prepared: (i) positive control (PC; 0.52% available P), a regular sow diet containing inorganic P, (ii) negative control (NC; 0.20% available P) without inorganic P, and (iii) NC + 500 units of phytase/kg diet. Each diet was fed randomly to 15 sows for 21 d (from 5 d prior to farrowing to 15 d after farrowing). At day 15 after farrowing, ATTD of P did not differ between PC and NC. Phytase supplementation to NC increased (P 0.05) ATTD of CP and Ca. On day 1 after farrowing, plasma P was 0.66 mmol/L lower (P < 0.05) in sows fed NC than PC. Phytase supplementation to NC increased (P < 0.05) plasma P by 0.40 mmol/L on day 1 but not on day 15 after farrowing. In conclusion, phytase supplementation increased P bioavailability in lactating sows.