In this study ten 56 day old barrows were surgically fitted with a postvalvular T-cecum cannula to evaluate the effect of microbial phytase on apparent and true ileal amino acid (AA) digestibility and nitrogen (N) utilization. It was found that the addition of phytase to a semipurified soybean meal-based diet did not affect the apparent ileal digestibility of several amino acids. Also, difference between the basal and homoarginine (HA) diets in nitrogen digestibilities indicated that the guanidination may limit the use of the HA method in determining endogenous proten losses