In plant-based feed ingredients there is a considerable amount of P bound to phytate, limiting the amount of P that is available for utilization, but inclusion of microbial phytase in pig diets increases the digestibility of P. The negatively charged phytate molecule can chelate Ca cations resulting in formation of insoluble Ca-phytate complexes. Degradation of phytate by microbial phytase may prevent formation of these non-digestible complexes, resulting in increased Ca digestibility. It is also possible that use of exogenous phytase reduces endogenous loss of Ca. If indeed the reduced endogenous loss of Ca is a result of degradation of phytate, it is expected that increased doses of dietary phytase will linearly reduce endogenous losses of Ca, but this hypothesis has not been experimentally verified. Therefore, this experiment was conducted to test the hypothesis that increasing dietary phytase reduces basal endogenous loss of Ca and increases digestibility of P in growing pigs.