The larvae of salamanders isolated from 19 pregnant females were grown to the middle of pre-metamorphosis. The siblings obtained from each single female were divided into two parts; larvae of one group were kept at 21 degrees C (control group), and those of the other--at 27 degrees C (experimental group). In all, 38 groups of experimental animals were employed. In animals kept at 21 degrees C the heat resistance of the organism, muscles and contractile muscle models were determined, whereas in those kept at 27 degrees C only the thermoresistance of muscles and their models only was registered. The time of loss of excitability in response to the electrical stimulus during heating in the Ringer solution at 36 degrees C served a criterion of heat resistance of muscles. The time of loss of contractility with the addition of ATP at 36 degrees C was a criterion of heat resistance of muscle models. The time of the onset of thermal shock at 34 degrees C being a criterion of heat resistance of the organism. It has been found that siblings showing a lower resistance level of muscles and of their models at 21 degrees C, when kept at 27 degrees C, increased their resistance much more as did siblings with the initially higher resistance. During the thermal selection, individuals of such initially less resistant families displayed selective advantages.