Grass carp introductions at Red Haw Lake, Iowa resulted in a decrease of aquatic macrophytes from 2,438 g/m2in 1973 to 211 g/m2in 1976, with species of Potamogeton, Elodea, Ceratophyllum, and Najas all controlled effectively by grass carp. During 1974-1976 mean nitrites, nitrates, biological oxygen demand, and turbidity showed significant decreases, while alkalinity increased significantly from a mean of 115 mg/liter in 1974 to 132 mg/liter in 1976. Mean concentrations of organic and inorganic phosphates gradually increased during the investigation, but were not statistically different. Average primary production was nearly identical in 1974-1975 at about 2 g carbon/m2/day, but decreased significantly to 1.35 g carbon/m2/day in 1976. Growth of stocked grass carp was rapidly increasing from a mean weight of 380 g in July, 1973 to 6,847 g by October, 1976. Body condition ranged from 1.05-2.02 with average condition over 1.37 in October and 1.25-1.30 in January-February. Greatest population biomass was estimated in 1975 at 61 kg/hectare. Grass carp consumed all major plant groups at the lake with greatest selection for Najas and Potamogeton. Movement, behavior, and activity as determined by ultrasonic telemetry showed grass carp inbabited all areas of the lake, but overall there was a preference for sballow areas of the main lake with lesser selection for embayments. Most of the time grass carp were sedentary near weed beds with more rapid and extended movement in midwater. Normal swimming speed in midwater was 0.12-0.35 m/s with maximum speed of 1.46 m/s. Homing tendency was shown in two of nine tagged fish. There was similarity in nocturnal and diurnal activity. Reduction in vegetation biomass by 91% in four years increased the opportunity for anglers to fish from shore. During the investigation popularity of shoreline fishing increased by 241% with catch success remaining greater than 0.70 fish per hour.