The effects of fish predation and food availability on population densities and demography of zooplankton were investigated in Dynamite Lake, Illinois, USA, a lake with a high density of size—selective planktivorous fish and low food levels. Fish predators (bluegill sunfish) and food levels (phytoplankton) were manipulated in replicated, factorial—design field experiments during two summers (1980 and 1981). Overall, population densities of zooplankton were affected much more by manipulations of food availability than by manipulations of fish predation. The cladocerans Bosmina longirostris, Ceriodaphnia lacustris, and Diaphanosoma birgei were greatly increased in density by elevated phytoplankton levels in both years, in the presence and absence of fish. Demographic analysis in 1981 revealed that increased densities in response to elevated food levels resulted from both an increase in birth rates (Bosmina, Diaphanosoma) and a decrease in mortality rates (Ceriodaphnia, Diaphanosoma). The rotifers Lecane and Monostyla also increased dramatically in response to elevated phytoplankton densities. Copepods were less responsive to manipulations of food levels, but several taxa exhibited increases in density in response to increased phytoplankton abundance. Few species were reduced in density by fish predation. Ceriodaphnia density was reduced by fish more than any other species in the entire community, and the density of even this species was much more affected by food availability. In terms of percent change relative to controls, increased food availability had much more of an effect than fish predation on the density of most zooplankton species and on total zooplankton abundance. Fish predation had several effects on the size structure and life history traits of the cladocerans. All three species attained larger sizes when fish were excluded than when fish were present. Cladoceran individuals also initiated reproduction at a smaller size and produced smaller offspring in the presence of fish. The reductions in mean body size, size at maturity, and offspring size in the presence of fish were most pronounced in Diaphanosoma and Ceriodaphnia, the two largest species. Smaller body size and size at maturity apparently allow the cladocerans to reproduce before reaching a size at which they become vulnerable to size—selective fish predators. Larger size at first reproduction and larger offspring size in the absence of fish may be a response to invertebrate predators, which assume more importance in the absence of fish and prey most heavily on smaller size classes. Flexibility in these life—history traits allows the cladocerans to withstand what appears to be intense size—selective predation by planktivorous fish.