The zooplankton community structure in Yeniçağa Lake was studied during an eighteen-month survey between October 1997 and August 1999. Eight Cladoceran species, eight Copepoda and six dominant Rotifera taxa were identified. Sampling at Yeniçağa Lake revealed large seasonal variations in the zooplankton abundance and succession. Before September 1998, the total zooplankton abundance ranged from 1,691 to 21,557 ind./m3. The dominant zooplankton groups changed from autumn/spring to summer: rotifers first increased, followed by nauplii in May, finally cladocera and calanoid copepods were dominant in the summer months. The summer population started with Daphnia magna and D. pulex, followed by the smaller Ceriodaphnia dubia. Diaphanosoma brachiurum started building up its population in the late summer. Copepoda were mainly represented by the calanoid copepod Acanthodiaptomus denticornis between May and September 1998. Zooplankton abundance and composition in the lake changed drastically after September 1998. Zooplankton abundance was between 237 (September) and 9,766 (August) ind./m3. Huge filamentous cyanobacteria blooms coincided with the depression of the zooplankton abundance. Microzooplankton (mainly Rotifera and Nauplii) were dominant and Rotifera were mainly represented by Keratella and Filinia. © Kasparek Verlag, Heidelberg.