Information on reproductive requirements of grass carp (Ctenopharyngodon idella Val.) was obtained from published accounts, from a visit to rivers in the USSR where introduced grass carp have reproduced, and from discussions with Soviet fishery workers. Turbulent areas at the confluence of rivers or below dams are the focal points for reproduction. Successful spawning occurs only in large rivers or canals where water velocity exceeds 0.8 m/s and volume is roughly 400 m3/s. The eggs are carried 50 to 180 km, depending on water temperature and current speed. The larvae hatch 1 day after spawning and make their way to vegetated lagoons, impoundments, or lakes closely connected to the river. They begin feeding on rotifers at 2 to 4 days and change to larger zooplankton in about a week. Temperatures required for stimulation of sexual maturation, egg incubation, and survival of young range from 19 to 30 C, with an optimum of about 23 C. Because requirements for each factor must be found in juxtaposition, successful reproduction occurs in only a few locations. Vulnerability to predators further checks population growth, once egg laying occurs. Although successful spawning of escaped grass carp in the United States is predicted, we believe that the resulting populations are likely to be small and to have little environmental impact except in local situations.