Claytor Lake, Virginia, an 1820 ha main stern hydroelectric reservoir, was studied for one year to determine the effect of water discharge on reservoir phytoplankton. Number/liter, areal units/liter, average cell size, and chlorophyll a content were used as measures of phytoplankton abundance. Rate of water discharge had an adverse effect on reservoir phytoplankton populations during spring and surruner. Although increased rate of water discharge caused increased phytoplankton discharge, the adverse effect of rate of water discharge on reservoir phytoplankton populations was probably at least partially due to additional discharge of nutrients. The inverse relationship between rate of water discharge and reservoir phytoplankton populations decreased in the uplake direction.