Zooplankton structure and dynamics were studied in a freshwater wetland subject to strong hydrologic fluctuations. This wetland underwent a six-year drought that terminated at the end of 1996. At that time, inundation area and water level increased, diminishing macrophyte cover in some cases and enhancing a continuing eutrophication problem. Sampling was performed monthly from January 1996 to December 1998 in five shallow water sites. Zooplankton were a mixture of limnetic and littoral species (66 rotifer, 15 cladoceran, and 10 copepod taxa, plus unidentified oligotrichid and peritrichid ciliates), the composition of which changed coincident with the shift of the hydrologic conditions. The most common taxa were bdelloid rotifers and Lecane closterocerca. The microcrustaceans Ceriodaphnia dubia, Daphnia curvirostris, and Acanthocyclops robustus were frequent. Ciliates were the most important component of zooplankton in terms of biomass, except in spring when cladocerans were dominant. Rotifer biomass was the lowest fraction of zooplankton. When community structure was described by taxonomic classes, mean biomass, or time biomass trajectories, only weak spatial patterns were found following the flux of water in the wetland. Each zooplankton group was distinctly affected by flood. Ciliates and rotifers increased biomass after the flooding, possibly as a result of the enhancement of eutrophication; cladoceran populations decreased after the flooding likely due to macrophyte loss that facilitated planktivorous fish control. Copepods increased biomass consequent to the stabilized water level. There was a clear seasonal trend of zooplankton biomass seemingly unaffected by flood. Ciliates and rotifers had fluctuating low biomass in winter-spring, with maximum biomass in summer-autumn; cladocerans appeared and peaked only in spring; copepods peaked in late winter and spring.