Forty Holstein male calves were used in 2 experiments to determine the P requirement. At 12 weeks of age the calves were changed to a semi-purified ration containing 0.09-0.10% P. Dicalcium phosphate was added to the basal ration to supply P levels of 0.09, 0.12, 0.18 or 0.30% in experiment 1 and 0.14, 0.22, 0.30 or 0.38% in experiment 2. Twelve different response criteria were used to assess the adequacy of P levels used and among these were feed intake and efficiency, body weight gains, serum inorganic phosphorus levels, serum phosphatase activity, bone ash and bone growth as measured from femur and rib autoradiographs. From these studies it is evident that the minimum P requirement of calves weighing from 200 to 275 lbs. at 12-18 weeks of age approximates 0.22% of the air-dried ration. Adding a factor of safety to this minimum value, it is recommended that 0.30% P be included in the rations of calves of the size and age used in this study.