Suitability of silver carp (Hypophthalmichthys molitrix) as a biological agent in controlling phytoplankton was studied in situ in 3 sequential dairy farm ponds in Anderson County, South Carolina. Concentrations of chlorophyll a, soluble orthophosphate and associated water quality parameters were measured biweekly from May 1979 through May 1980 prior to introduction of silver carp (100-150 mm total length; 23.1 g average weight) to 1 pond on 12 June 1980. Water chemistry measurements continued through October 1980. Increases of chlorophyll a concentrations in 1980 over 1979 values were 5 times to 7 times in ponds without silver carp and only 2 times in the pond with silver carp. In addition, the pond with silver carp showed significantly lower total hardness and calcium concentrations during 1980. These data suggest that silver carp can effectively lower phytoplankton biomass. Silver carp were harvested during December of 1980. Fish averaged 403 mm in total length and 690 g in weight. Net production for the 180-day period was 2,507 kg/ha.