The influence of microbial phytase on tibial ash content, serum calcium, inorganic phosphorus and alkaline phosphatase levels and processing yields were investigated using day old commercial broiler chicks. Tibial ash, serum calcium and serum inorganic phosphorus were significantly (P < 0.01) increased by the addition of 750 and 1000 units of dietary phytase / kg in low available phosphorusdiets both at sixth and eighth week of age. The serum alkaline phosphatase level was significantly (P <

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... 1) decreased in the diets supplemented with phytase. upplementation of phytase had no effect on per cent dressed yield and giblet yield. The per cent ready-to-cook yield was significantly (P < 0.05) higher in groups fed standard diet, low available phosphorus diet and 500 units phytase supplemented diet at sixth week, whereas at eighth week it was significantly (P < 0.05) higher in groups fed standard diet and 1000 units phytase added diets.