The zooplankton assemblage of ionically polluted, culturally eutrophic Onondaga Lake was monitored over the 1979–1989 interval, and compared to surveys conducted in 1968 and 1978. A major shift in the assemblage was apparent by 1987, soon after the closure (1986) of an industrial discharger of ionic (Cl; Na+and Ca2+) waste. Species richness increased from 8 to 18 common species, and more efficient grazers, large-bodied cladocera and the calanoid copepodDiaptomus sitihides, became dominants. Until 1987, a single cyclopoid copepod,Cyclops vernalis, was the dominant component of zooplankton biomass. The most likely cause for the shift in the zooplankton assemblage of the lake is the reduction in salinity, and attendant precipitation of calcium carbonate, associated with the closure of the industry. Improved clarity in the lake, manifested largely as intervals of dramatic increases described as ‘clearing events’, observed annually since 1987, has been attributed to die shift to more efficient grazers. The increased grazing pressure, particularly from large daphnids, may also be responsible for the return of late summer nuisance blooms of filamentous cyanobacteria, not observed in the lake since the early 1970s.