The zooplankton community of the Ogochi Reservoir was investigated with brief field experi- ments to examine the mechanisms that maintain a stable horizontal distribution of species in the warm-water season. Daphnia gale&a 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. gale&a 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 hor- izontal pattern in the community structure. Although horizontal variation in the abundance of freshwater zooplankton can be commonly found on various scales (e.g. Nie et al. 1980), its ecological implication has been little studied from the aspect of community structure (Pinel-Alloul et al. 1988). This situation is due partly to an impression that, in contrast to terrestrial habitats, the pelagic one provides a ho- mogeneous environment for planktonic or- ganisms or that heterogeneity in the envi- ronment is ephemeral. Lewis (1978) recognized, however, that horizontal vari- ation in the abundance of zooplankton can be divided functionally into two patterns: