The zooplankton community of the Ogochi Reservoir was investigated with brief field experiments to examine the mechanisms that maintain a stable horizontal distribution of species in the warm-water season. Daphnia galeata predominated near the dam site, and its abundance declined toward the headwaters. The abundance of other species increased away from the dam, and Bosmina longirostris was found abundantly at the site near the headwaters. Demographic analysis and field experiments demonstrated that D. galeata had the potential to depress the abundance of other species through exploitative competition and to predominate at sites near the headwaters as well as at the dam site. This potential was never realized, however, at a site near the headwater because of a high mortality rate that could not be explained by food abundance or physical and chemical conditions in the water. The invertebrate predator Leptodora kindtii was abundant at sites nearer the headwaters, but its occurrence was limited temporally. Trammel gill-net samples revealed that planktivorous fish, which prey selectively on the superior competitor D. galeata and the invertebrate predator L. kindtii, were distributed toward the headwaters in the warm-water season. The high mortality rate of D. galeata at the site near the headwaters apparently was attributable to predation by fish. The data presented demonstrate that the pelagic environment in the reservoir is functionally heterogeneous and that species interactions play an important role in maintaining the stable horizontal pattern in the community structure.