Twenty-seven shallow tilapia culture ponds, having fish densities of 0.1 to 12 individuals · m−2were studied in Guanacaste Province, Costa Rica. Temporal changes in plankton community structure following the emptying and subsequent filling of each pond were noted. In general, newly filled ponds were initially colonized by microflagellates, which in turn were replaced by green and finally blue green algae.Microcystis aeruginosafrequently became the dominant phytoplankter. Its density was correlated with both high tilapia densities and low zooplankton concentrations. When tilapia densities were maintained below 0.5 fish · m−2, the copepodMesocyclops leukartiidominated the zooplankton community, and algal biomass was moderate. At higher tilapia densities, rotifers replaced copepods as the dominant zooplankters. When tilapia densities exceeded two adult fish m−2, negligible zooplankton levels were attained, and phytoplankton densities reached their highest levels (>2 × 10 cells · ml−1). At these high algal densities, nighttime dissolved oxygen concentrations were so low that the tilapia were forced to the water's surface to breathe. As a result, tilapia predation by fish-eating birds was significant.