Phytate is the main storage form of phosphorus in many plant seeds, but bound in this form it is a poor nutrient for monogastric animals.  Its availability can be significantly increased by the addition of the enzyme phytase, which releases phosphate from the substrate, phytate.  We have engineered a phytase from Aspergillus niger in tobacco seeds, providing a stable and convenient packaging of the enzyme that is directly applicable to animal feed.  The enzyme was expressed a 1 percent of the soluble protein in mature seeds.  In in vitro tests that simulated chicken crop and stomach conditions, release of phosphate from feed by addition of transgenic seeds was demonstrated.  supplementation of broiler diets with transgenic seeds resulted in an improved growth rate, comparable to diets supplemented with fungal phytase or phosphorus.  Addition of phytase-transgenic seeds to animal feed thus obviates the need for inorganic phosphorus supplementation, and is environmentally desirable because of the reduced excretion of phosphorus.