Corn is the primary grain used in pig diets and provides most energy to the diets. Because energy is the most expensive component in diet formulation, it is critical to accurately determine energy concentrations in corn. Use of exogenous phytase in pig diets has been a standard and most phytase is expected to generate extra-phosphoric effects that result in increases in minerals, amino acid, or energy digestibility. Therefore, phytase companies have provided customers with matrix values that can be used for down specs of energy and nutrients in diet formulation. It is thus important to confirm if dietary phytase releases energy and other nutrients as suggested before using them. To our knowledge, however, there is no information on how much phytase can increase net energy (NE) in corn when fed to group-housed pigs. Therefore, the objective of this experiment was to test the hypothesis that addition of microbial phytase to a corn-based diet increased the apparent total tract digestibility (ATTD) of gross energy (GE) and concentration of NE by group-housed growing pigs.