Semi-purified diets containing 19.6 MJ digestible energy/kg dry matter were fed to groups of rainbow trout initially weighing, on average, 53 g/trout. In a series of 12 diets, mono sodium phosphate replaced a silica carrier aiming at a range in P concentration from 1.0 to 10.9 g/kg dry matter. Each diet was fed to a group of 20 trout. During the trial of 53 feeding days, response of dry matter intake, weight gain, feed conversion ratio and P concentration of the gain followed exponential functions. For achieving 95 % of the potential maximum gain, a dietary concentration of 3.7 g P per kg dry matter was required. However, 5.3 g P per kg dry matter were required for achieving high P concentration in the gain. P was utilized most efficiently at a dietary concentration of 2.5 g/kg dry matter. A dietary P concentration of 0.25 g available P per MJ digestible energy was recommended to be sufficient for trout diets.