The residual phytase activity, phytic acid P content, and microstructure of the digesta along the gastrointestinal tract (GIT) of broiler chickens fed a corn expressing microbial phytase was studied in a 14-d experiment. The phytase activity of the corn expressing phytase (CBP) was determined to be 660 phytase units/g and was incorporated into broiler diets at varying rates. One hundred forty-four 7-d-old male broiler chickens were grouped by weight into 8 blocks of 3 cages with 6 birds per cage. Three dietary treatments were randomly allotted to the cages within blocks. The corn-soybean meal-based diets consisting of low P and Ca (no added inorganic P) supplemented with 0 (control), 55, or 550 g/kg of CBP (substituting corn) were formulated to contain 0 (control), 36,300, or 363,000 phytase units/kg of phytase activity, respectively. Birds were fed the dietary treatments for 14 d when they were killed and digesta samples from the proventriculus and gizzard, jejunum, and ileum were collected. The residual phytase activity along the GIT increased linearly and quadratically (P < 0.01) with the addition of CBP to the control diets. There was a decrease (P < 0.01) in residual phytase activity as digesta moved distally along the GIT with CBP supplementation. Phytic acid P content significantly decreased (linear, P < 0.01; quadratic, P < 0.05) with CBP supplementation of the control diets. There was substantial degradation (linear and quadratic, P < 0.01) of phytic acid content caudally along the GIT of birds. Extensive cell wall degradation of digesta samples from the proventriculus and gizzard in broilers fed the highest level of CBP compared with birds fed the control diets was observed. The addition of CBP to control diets led to a rapid degradation of the cell walls of digesta and a marked reduction in phytic acid P concentration of digesta in broiler chicks.