Water quality parameters and phytoplankton in ponds stocked with grass carp, Ctenopharyngodon idella (test ponds) and in control ponds were compared during the summers of 1975 and 1976. Grass carp rapidly consumed available aquatic vascular plants. Dissolved oxygen levels in test ponds remained high as long as bottom growths of the filamentous green alga, Spirogyra sp. were present, but consumption of this alga in addition to vascular vegetation led to significantly lower dissolved oxygen concentrations in the water directly above the sediments. The most consistently affected parameters were turbidity and potassium concentrations, both of which were significantly higher in test ponds. Phytoplankton populations consisting primarily of dinoflagellates were not affected by grass carp introduction. As much as 54% of the phosphorus and 42% of the nitrogen released by consumption of plants were incorporated into new fish tissue. Most of the phosphorus not taken up by the fish was sequestered into components other than water or phytoplankton. Estimated potassium incorporation into fish tissue was low (2.5% or less), and recovery in water was approximately 2-to 6-fold higher than phosphorus. Potassium in the water appeared to be an excellent indicator of vegetation consumption by grass carp.