Heat resistance of the gastrocnemius muscle was studied in five species of the Rana esculenta complex. It was similar in R. bedriagae, R. lessonae, and in the European form of R. ridibunda; while North African R. saharica demonstrated a lower heat resistance. No heterosis was expressed in R. esculenta, a clonal hybrid of R. lessonae and R. ridibunda, for the heat resistance of the muscle. Moreover, this species demonstrated low heat resistance at the highest test temperatures (42 degrees C). Comparison of diploid and triploid R. esculenta syntopically occurring in the same water bodies demonstrated no heat resistances between them, thus, suggesting that polyploidy has no effect on this parameter at least in this case.