In experiments with two closely-related species of soil nematodes C.elegans and C.briggsae it was shown that resistance of cholinergic synapses correlates with differences in behavior thermotolerance of nematodes. The increase in thermotolerance of cholinergic synapses’ functions may be the cause of adaptive changes of thermotolerance of organisms of nematode of Caenorhabditis genus in the course of their evolution.