Bream (Abramis brama orientalis) inhabitating the part of the Ivan'kovo Reservoir directly exposed to effluent from the Konakovo Hydroelectric Power Plant are smaller and become sexually mature sooner than elsewhere in the reservoir. Moreover, they have longer tail peduncles and anal, thoracic, and dorsal fins than other bream. And, in contrast with the latter, many (20 percent) have two rows of pharyngeal teeth. Thus, the differences in ecological (growth rate, condition factor, spawning times, etc.) and meristic and plastic characters (branched rays, enlarged fins, etc.) as well as in composition of parasites point to the development of an isolated bream population.