Experiments to determine the phosphorus requirement for fattening lambs have been carried out employing 675 western lambs in a series of five experiments. Lambs fed on rations containing 0.07 to 0.12 per cent phosphorus or an intake of 1.20 to 1.67 grams of phosphorus per 100 pounds live weight showed abnormally slow gains, high feed requirements, and specific symptoms of aphosphorosis such as depraved appetite, unthrifty appearance and low blood phosphorus. A level of 0.14 per cent phosphorus in the diet indicated a borderline phosphorus deficiency, which proved to be inadequate for normal growth and feed utilization. Lambs receiving rations with a phosphorus content ranging from 0.15 to 0.23 per cent were adequately supplied with phosphorus as indicated by good rate of gain, efficient feed utilization and normal blood phosphorus levels. On the basis of phosphorus intake per unit live weight, lambs receiving from 2.17 to 3.38 grams daily per 100 pounds of live weight will meet their phosphorus requirement. Although a somewhat lower phosphorus level may be adequate, due to the different conditions under which lambs are fattened, it is suggested that the optimum feed phosphorus requirement for fattening lambs be set at 2.40 grams per 100 pounds of live weight or the ration should contain on a dry basis 0.17 per cent or more phosphorus. These results indicate that a phosphorus deficiency in lambs exerts a more depressing effect on the utilization of the feed than on the appetitie.