Phosphorus absorption in the young turkey, measured with the aid of yttrium-91 as a reference substance, was linear with phosphorus intake. This observation suggested the lack of adaptation of the phosphorus transport mechanism to changes in the phosphorus needs. Phosphorus retention and bone ash initially increased with increased intake to plateau at a dietary phosphorus concentration of about 0.8%. The excess phosphorus absorbed from diets higher in phosphate, was eliminated in the urine. Plasma inorganic phosphate was linear with absorbed phosphate. The apparent phosphorus absorption from the basal vegetable diet was 45% of the intake and that of the inorganic supplement, sodium phosphate and calcium monophosphate, was about 100%, at low calcium intakes. Increasing the calcium intake above 440 mg/day progressively depressed the absorption of phosphate. This inhibitory action of calcium on phosphate absorption was resolved into a linear coefficient.