Phytic acid or phytate is a naturally occurring organic complex found in plants. As a reactive anion, it forms a wide variety of insoluble salts with divalent and trivalent cations. Phytic acid is also known to complex with proteins and consequently reduce their availability. Recent studies indicate that phytic acid reduces the activity of pepsin, trypsin and αamylase. Because of a lack of endogenous phytase, which hydrolyses phytic acid, phytate phosphorus is biologically less available to poultry. Because of the complex factors influencing phytate hydrolysis, such as dietary calcium content, inorganic phosphorus and vitamin D3, and the age and genotype of birds, there is wide disagreement concerning the ability of poultry to utilize phytate phosphorus. Data suggest that the amount of endogenous phytase is extremely low in young birds but that it increases with age. Cereal based poultry diets supplemented with microbial phytase result in increased digestibility and availability of phytate bound phosphorus, calcium, zinc and copper. Microbial phytase supplementation has also been shown to increase ileal digestibility of crude protein and amino acids in female broiler chickens and in female turkeys, but curiously not in male chickens. There is no report to date of such a study in male turkeys. While the efficacy of supplemental microbial phytase depends on its rate of inclusion, on the calcium and phytate phosphorus contents and on the dietary ca1cium:phosphorus ratio, clear benefits have been shown in terms of increased availability of phytate-bound minerals and crude protein, and reduced environmental pollution through lower levels of phosphorus and nitrogen excretion. To maximize the benefit from the addition of microbial phytase, future research should focus on determining the optimum dietary conditions for it to work.