The distribution of fish and macroinvertebrate populations near two electrical generating stations located on the Wabash River were examined during the summer and early fall for six years. Segments of the river were subdivided into zones corresponding to the imposed thermal conditions and to different kinds of habitat. Fish responded readily to changing thermal conditions and concentrated in areas of the river which provided temperature close to their optimum. Although most species reacted behaviorally to the altered thermal conditions, only two groups appeared to be affected permanently as populations. Flathead catfish (pylodictis Olivaris) responded to higher temperatures with enhanced reproductive success which increased population density two or three times. Redhorse (Moxostoma erythrurum and M. breviceps) are more thermally sensitive and population densities are likely to be reduced with higher temperatures. Complicating factors include the fact that several species of fish, including redhorse and sauger (Stizostedion canadense), appear to rely upon recruitment from tributaries to sustain the mainstem populations. Long-term changes in the density of other species were not detected.