Zooplankton were obtained in the U.S. waters of Lake St. Clair at 28 stations sampled monthly from July through September 1973. Distribution and abundance of zooplankton were compared with physicochemical data collected concurrently and with previous studies on Lake St. Clair zooplankton. Sixty five species of Rotifera were recorded. Brachionus angularis, Conochilus unicornis and Synchaeta stylata were the three most abundant species. Total rotifers, averaging 47.1 individuals per liter over the study period, were about 30 times more abundant than the micro-crustaceans. The Cladocera were represented by 28 species, while five species of cyclopoids, seven calanoids and four harpacticoids were recorded from the Copepoda. Bosmina longirostris was the predominant micro-crustacean. Spatial distribution of zooplankton was hypothesized to be primarily dependent on current patterns generated by prevailing winds. The Canadian portion of the lake has a persistent eddy structure which apparently allows build-up of nutrient levels encouraging high zooplankton biomass. The U.S. side, however, experiences more rapid flow-through of St. Clair river water which accounts for lower zooplankton abundance. Despite the fast flushing rate on the U.S. side, a localized patch of high zooplankton density and a community structure indicative of eutrophic situations was consistently observed off the Clinton River Cutoff Canal. This is presumably due to nutrient loading from the Clinton River watershed.