Zooplankters were sampled from Lake Fayetteville and the Illinois River from March 1992 through March 1993. Zooplankton densities were significantly greater in the lake habitat than in the stream. Mean annual density was 633 ± 86/L in Lake Fayetteville, compared with 9 ± 1/L in the Illinois River. Significant seasonal differences in density were also observed. Maximum density was observed during fall, and minimum density during spring. There was also a significant two way interaction between habitat and season. This interaction indicated a much greater seasonal variation in zooplankton density in Lake Fayetteville than in the Illinois River. Depth distribution also was different between the lake and stream habitats. In Lake Fayetteville zooplankters were most abundant in the upper three meters of the water column, and abundance decreased significantly with water depth. The opposite was true of zooplankton abundance in the Illinois River. Plankters were most abundant nearest the bottom, and plankton density decreased significantly closer to the surface.