The experiment was conducted to evaluate the sparing effect of microbial phytase on the need for dietary zinc supplementation in chicks. A maize-soya-bean meal basal diet, containing 33 mg of zinc and 16 mg of copper per kg, supplemented with 0, 6, 12, 18, 24, 30 or 60 mg of zinc as sulphate per kg or with 250, 500, 750 or 1000 units (FTU) of microbial phytase (3-phytase from Aspergillus niger, Natuphos R ) per kg was given to 1-day-old chicks for 20 days. Sixteen chicks placed in individual cages were assigned to each diet except the unsupplemented basal diet which was assigned to 32 cages. Actual range of phytase supplementation was 280 to 850 FTU per kg diet. Growth performance was not affected by microbial phytase. Chicks given the unsupplemented basal diet and the basal diet supplemented with 60 mg of zinc per kg displayed similar performance. Bone weight, bone ash, liver weight and liver dry matter were independent (P . 0.1) of zinc and phytase supplementations. Plasma, bone and liver zinc concentrations increased linearly (P , 0.001) and quadratically (P , 0.001; P , 0.001 and P , 0.05, respectively) with zinc added. Plasma zinc tended to increase linearly (P 5 0.07) and bone zinc increased linearly (P , 0.01) with phytase added but no quadratic response was detected (P . 0.1). Liver zinc was unresponsive to phytase added (P . 0.1). Liver copper decreased linearly (P , 0.001) and quadratically (P , 0.01) with zinc supplementation. Mathematical functions were fitted to the responses of plasma and bone zinc to zinc and phytase added and used to calculate zinc equivalency values of phytase. The models included a linear plateau response to zinc added and a linear response to phytase added. In diets without phytase, plasma and bone zinc concentrations were maximised for a dietary zinc concentration of 55 and 51 mg/kg, respectively. Over the range of 280 to 850 FTU, 100 FTU was equivalent to 1 mg of zinc as sulphate. Consequently, in a maize-soya-bean meal chicken diet formulated to contain 60 mg zinc per kg, zinc ingested, and in turn, zinc excreted may be reduced by around 10% if the diet contains 500 FTU as Natuphos R per kg.