It was shown that short-time periodical temperature fluctuations within the upper temperature zone suboptimal for the growth of juvenile rainbow trout Parasalmo (Oncorhynchus) mykiss exerts a favorable effect on the growth and energetics of fish. Under variable temperature conditions, an increase in the rate of growth and improvement of energetic and production indices of juvenile trout, as compared to constant thermal conditions optimal for growth, are observed. Efficiency of food conversion is increased, and oxygen expenditure for the gain of body weight unit is decreased. Optimization of life activity and energetics of rainbow trout observed under variable temperature conditions indicates the need for subsequent discussion and clarification of the concept of an ecological optimum with respect to the temperature factor for fish and other poikilothermal hydrobionts.