Two experiments were conducted with rainbow troutOncorhynchus mykissto investigate the influence of several diets and supplemental phosphorus on discharges of phosphorus in hatchery effluent water. A diet was formulated to contain no fish meal and a reduced level of non-phytin phosphorus (approximately 0.9%) provided, in part, by supplemental defluorinated rock phosphate, which has a low solubility in water. Feeding rainbow trout this diet supported 86% of the growth attained with a conventional hatchery diet while discharges of phosphorus in the effluents decreased by 40–51%.