The population behavior of Daphnia gessneri Herbst, 1967 in a floodplain lake (Lago Grande) of the lower Rio Solimoes was investigated between April 1979 and March 1980 with regard to 1) predation by the fish called tambaqui (Colossoma macropomum, Characidae), 2) water level fluctuation and 3) water transparency. Zooplankton density samples were collected at two sites near mid-lake, where water depth and Secchi disc transparency were measured. I n addition, qualitative samples of zooplankton and fish collections were taken at several sites in the adjacent floodplain areas. The author concludes that fluctuations in Daphnia gessneri populations correlate most with intense predation by fish and water turbidity.