Ninety-six crossbred pigs with an average initial weight of 10.3 kg were used in a 4-wk experiment to investigate the P equivalency value of microbial phytase in weanling pigs using performance, rib mineralization and faecal digestibility measurements. A 19% CP, maize-soyabean meal basal diet low in P (0.35%) and Ca (0.50%) was fed. Diets 1, 2, 3, and 4 contained 0, 167, 333, and 500 units (U) of added Natuphos® phytase per kg of diet, respectively. Diets 5, 6, and 7 contained no added phytase, and 0.40, 0.45, and 0.50% P, respectively. Body weight and pen feed consumption were measured weekly. During wk 4, pen faecal samples were collected twice daily for 5 d for determination of P, Ca and DM digestibilities. At the end of wk 4, the barrow from each pen (n = 48) was killed for collection of tenth ribs for determination of rib shear force, and ash content. Adding phytase to low P diets linearly increased (P < 0.02 to 0.001) ADG, rib shear force, rib ash weight and ash percent. Added P linearly increased (P < 0.02 to 0.001) ADG, rib shear force, rib ash weight, and ash percent, Ca and P digestibility and digestible Ca and P. Based on phytase and P linear or nonlinear response equations for ADG, P digestibility, rib ash weight and rib shear force, 500 U/kg of microbial phytase was equivalent to 1.03, 0.78, 0.89, and 0.69 g of inorganic P, respectively. The average equivalency of 500 U/kg of phytase was 0.84 g of P per kg of diet.