Two experiments involving 115 pigs were conducted to assess the efficacy of a microbial phytase (Allzyme Phytase; Alltech, Nicholasville, KY) produced by Aspergillus niger in low-P, corn-soybean meal-based diets. The phytase supplement contained 50 phytase units/g and 1.43% P. In Exp. 1, growing-finishing pigs were fed fortified corn-soybean meal diets formulated to be adequate (.50%) or inadequate (.30%) in P during the growing phase (38 to 57 kg BW) followed by adequate (.40%) or inadequate (.30%) P, respectively, during the finishing phase (to 101 kg BW). Dicalcium phosphate was the source of supplemental P. Half the diets were supplemented with phytase at 500 phytase units/kg. Rate and efficiency of gain and bone breaking strength were decreased (P < .01) when the low-P diet was fed. Adding phytase to the low-P diet restored performance and bone breaking strength (P < .01) to levels that approached those of pigs fed the adequate-P diet. In Exp. 2, growing pigs (13 kg BW), were fed a low-P (.32% total P; .048% available P) based diet supplemented with graded levels of monosodium phosphate to provide 0, .075, and .15% added P or with phytase to supply 250, 500, 1,000, or 2,000 phytase units/kg. Chromic oxide was included as an indigestible marker for determining apparent absorption and fecal excretion of P. Performance and bone strength increased linearly with added monosodium phosphate (P < .01) and with increasing levels of supplemental phytase (P < .05). A portion of these increases from phytase was attributed to the P supplied by the phytase mix (.007, .014, .028, .057%, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)