We sampled zooplankton from mid-May to early September over 2 years to study the spatial and seasonal distribution of animals in a large urban wetland of Lake Ontario. Samples were from several habitat types including open water, vegetated areas, fast-flow areas, and a sewage lagoon. Mean seasonal densities ranged from 17 individuals/L (5 µg/L) in fast-flow areas to 1800 individuals/L (1100 µg/L) in low-flow, highly vegetated areas. All of our sites were dominated by herbivorous rotifers (e.g., Brachionus sp., Polyarthra sp., Keratella sp.), small herbivorous cladocerans (Bosmina longirostris; mean length <300 µm), cyclopoid nauplii, and medium-sized cladocerans (e.g., Moina micrura; mean length 300-600 µm), which were absent from the most eutrophic sites. The high levels of inorganic suspended solids in the marsh appeared to select against large filter feeders such asDaphnia and allowed smaller zooplankton to dominate. Multivariate analyses indicated that the zooplankton distribution was related to flow rate, extent of macrophyte cover, and level of site degradation. If the forthcoming carp (Cyprinus carpio) exclusion from Cootes Paradise Marsh results in increased macrophyte growth, we predict that zooplankton biomass will increase and that the zooplankton community may shift to larger forms.