A study of the availability of phytin phosphorus was made with the rat using diets of normal calcium and phosphorus levels. About 85% of the dietary phosphorus was in the form of phytic acid and its salts, as carried by wheat bran. In the absence of vitamin D, the phytin phosphorus of wheat bran was poorly utilized. An adequate intake of this vitamin increased the utilization nearly to that of inorganic phosphorus as measured by bone ash. The presence or absence of phosphorus splitting enzymes in the diets did not alter the apparent availability of phytin phosphorus.