Temperature election and avoidance are investigated in Simocephalus vetulus (O.F. Müller 1776), caught during the vegetative season in the littoral of the Rybinsk Reservoir. The methodology for determining a thermotolerance range, a temperature zone of normal vital activity and pessimal temperatures, based on the data analysis of distribution of cladocerans in a thermal gradient installation, is proposed. The range of final elected temperatures (FET) (13.4–25.4 ^оС), corresponding to the optimum temperatures for growth and development of cladocerans, is indicated. The temperature zone of normal vital activity (>7.0 and <29.0 ^оС), pessimal temperature range (4.0–7.0 ^оС and 29.0–32.0 ^оС) and temperature limits of species tolerance (4.0–32.0 ^оС), coinciding with the field observations of S. vetulus populations, are calculated.