An analysis of data from four turbid North Carolina reservoirs showed that both the summer mean biomass and relative biomass of planktonic blue-green algae (Cyanophyta/Cyanobacteria) are frequently lower than would be predicted by models developed from natural north temperate lakes. These results are consistent with the hypothesis that the growth of blue-green algae in these reservoirs is reduced in the presence of high concentrations of non-algal turbidity.