In biological system, hydrolysis of phytic acid (the principle storage form of phosphorus in legumes, cereals, oil seeds and nuts) to myo-inositol and inorganic phosphate is an important reaction for energy metabolism, metabolic regulation and signal transduction pathways. The reaction was primarily catalysed by phytases (myo-inositol hexakisphosphate phosphohydrolase). Phytases are histidine acid phosphatases (HAPs), a subclass of phosphatases, which catalyze the hydrolysis of phosphate moieties from phytic acid, thereby, resulting in the loss of ability of phytic acid to chelate metal ions. The supplementation of animal feed with phytase increases the bioavailability of phosphorus in monogastric animals besides reducing the phosphorus pollution in the areas of intensive livestock units. Phytases can be derived from a host of sources including plants, animals and microorganisms, however, microbial sources are more promising for the production of phytases on a commercial scale. Of the various organisms reported, phytase production by Aspergillus sp., have been most commonly employed using either solid-state fermentation (SSF) or submerged fermentation (SmF) processes. The physicochemical characteristics and catalytic properties of phytases from various sources indicated it to be ester-hydrolyzing enzyme with an estimated molecular weight of 35-700 kDa depending upon the source of origin and are usually active within a pH range of 4.5-6.0 at 45-60degreesC. Generally, the phytases from bacteria have optimum pH in neutral to alkaline range while in fungi optimum pH range is 2.5-6.0. Phytases are fairly specific for phytic acid under the assay condition and it is possible to distinguish phytase from acid phosphatase that is incapable of degrading phytate. In this review, recent trends on the production and properties of phytases from various sources are summarized. (C) 2004 Elsevier Inc. All rights reserved.