Fingerling rainbow trout Oncorhynchus mykiss with initial mean weights of 9 g (small fish) and 35 g (large fish) were fed diets adequate in all known nutrients except phosphorus (P). In two experiments, triplicate lots of rainbow trout were fed basal diets containing either 0.14 or 0.41% non‐phytin P, with or without graded levels of supplemental P. Deficiency of P reduced growth, feed efficiency (weight gained/weight fed), bone ash, and whole‐body ash contents. The requirement for non‐phytin P by small trout for maximum growth and feed efficiency was not more than 0.41% of diet: The requirement by large trout was between 0.34 and 0.54% of diet. The requirement of non‐phytin P for maximum bone ash development was about 0.51% of diet for small trout and more than 0.54% for large trout. Whole‐body phosphorus content of small trout suggested a requirement above 0.51% but not more than 0.61% non‐phytin phosphorus. Although an effect of size of trout on the requirement was not clearly demonstrated, these results show that trout required more dietary P for bone mineralization than for weight gain. The minimum dietary requirement for non‐phytin P for bone mineralization was probably between 0.54 and 0.61% of diet. Discharges of P into effluent water increased significantly as trout were fed increasing levels of P. When trout were fed 0.61% available P, approximately 67% of P consumed was retained, and discharges of soluble P in effluents were 2.0 g P/kg weight gain or 1.8 g P/kg feed fed.
