The effects of a microbial phytase derived from Aspergillus niger (Natuphos®, BASF AG Ludwigshafen, Germany) on growth, body mineralization, phosphorus retention and phosphorus excretion were investigated in common carp (Cyprinus carpio L.). The test diet used in this study was based on soybean meal, fishmeal and wheat starch resulting in a total P content of 7.3 g/kg dry matter (DM). This diet was supplemented with two levels of monocalciumphosphate (MCP) and additionally with phytase at levels of 500 and 1000 U/kg. Triplicate groups of fish (10 carp/tank) of mean initial weight of 40 g were fed at a level of 20 g/kg W0.8 d−1 for 9 wk at a water temperature of 23°C. There was enhanced weight gain, crude ash content and P content in whole body with increasing dietary P content and phytase supplementation, respectively. The P gain was about 4 g/kg gain when P supply was adequate. Supplementation with phytase improved utilization of native plant P and reduced the P excretion/kg gain significantly by 30% compared with MCP-supplemented diets. This effect of the phytase was confirmed by measurements of the total P digestibility.