The southern part of the Kuril Islands is typical of an island arc. The structure of the earth's crust in that region has been well studied by seismic sounding. The present paper gives a short synthesis of data about the properties of the mantle obtained here during the detailed seismological observations in 1958–1962. The upper mantle under the belt of the Kuril volcanoes has small speeds of the P and S waves, reduced density, higher temperature and greater absorption of the S waves, which is undoubtedly connected with volcanic activity. Detailed seismological investigations were carried out on the southern part of the Kuril Islands from 1958 to 1962 (Fedotov et al., 1961). The investigations were carried out with the aim of obtaining accurate data on the crust and upper mantle properties and seismicity of this typical island arc area.