The herbivorous grass carp fed winter elodea exhibited a negative nitrogen balance in metabolism, suggesting that food having a higher protein content would be required for normal growth. About a third of the phosphorus contained in food was retained; hence, a combination of aquatic plants and grass carp might afford an effective biological method for removing phosphorus from water. Water quality was altered by the excretion of ammonium and orthophosphate by grass carp. Even though the grass carp might be useful in removing some phosphorus from water, its principal impact was to increase the rate of recycling of nitrogen and phosphorus.