The ultrastructure of the myocardial cells of the left and right ventricles and of the interventricular septum of Altai and Pamir yaks, permanently living at altitudes of 3000–3600 m above sea level, was studied. Electron-microscopic investigation of the myocardial cells revealed myocardia with a special ultrastructure of their cristae as well as ordinary mitochondria. In some groups of mitochondria the cristae consisted of polyhedral undulating membranes, and in other groups as polyhedral reticular structures. A large accumulation of glycogen granules was discovered beneath the sarcolemma, in the perinuclear zone of the crytoplasm, and between the myofibrils. The results indicate that mitochondrial cristae, twisted into a certain structural orderliness, leading to an increase in their number and useful area, ensures the working efficiency of the mitochondria. The large accumulation of glycogen granules in most of the myocardial cells evidently helps to maintain the energy potential of the myocardium and prevents it from developing hypoxia.