The values of the upper lethal temperature in young Prussian carp, common carp and Amur sleeper – the most thermophilic fish species living in freshwaters of Northwest Russia, were experimentally established. The initial fish acclimation temperature ranged from 4 to 32 °C. The critical thermal maximum and the lethal temperature values in all the investigated species increased at higher acclimation temperatures. At acclimation to a minimal temperature of 4 °C the most resistant were the Prussian carp and Amur sleeper; at acclimation to an abnormally high temperature of 32 °C the highest values of the critical thermal maximum and the lethal temperature were observed in Prussian carp and common carp. The critical thermal maximum increased by an average of 0.43 °С with every °C of rising acclimation temperature. The resultant values of upper lethal temperatures in juveniles of the given fish species can be used in developing the criteria for thermal pressure on freshwater bodies in areas with hot water plumes from heat and nuclear power stations, as well as where abnormal high temperature occurs in natural waters.