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An experiment was conducted to determine whether phytase additions to swine growing-finishing diets improved the availability of lysine, calcium, energy and phosphorus in corn and soybean meal-based diets. Diets investigated for growing-finishing barrows of high-lean-gain potential included: 1) University of Nebraska recommended diet formulations; 2) diets formulated to contain 85% of the lysine recommended; 3) lysine-deficient diets formulated with phytase; 4) phytase-formulated diets with phytase deleted; and 5) phytase-formulated diets with phytase deleted and with additional calcium and phosphorus. Pigs fed diets formulated to 85% of the recommended lysine level had slower growth, slower daily lean gain, poorer feed conversion, and less carcass lean. There was no effect of phytase addition or deletion on growth, feed efficiency, or carcass lean. There was no effect of phytase addition or deletion on bone strength or bone ash. These results do not agree with the large body of evidence regarding the improvement in availability of phosphorus in corn and soybean meal with the addition of phytase to swine diets and may be related to the growth impairment associated with the lysine deficiency.