Entirely new data have been obtained on the South Kuril environments at the time of the Middle Pleistocene warming. These data are correlatable with MIS 7. The data show climatic rhythms, and shed new light on paleo-oceanologic conditions and the evolution of environments at that time. Paleo-environmental reconstructions are based on multidisciplinary studies of the marine deposit sequence exposed in a sea cliff in the eastern part of Tanfiliev Island, including a buried oyster reef. In the course of the studies, analyses of mollusc fauna, diatoms, and pollen were performed. Benthic foraminifera were first found in the Middle Pleistocene sediments of the South Kurils. The sediments were dated using the 230Th/U method, and by the presence of the zonal diatom species Thalassiosira nidulus. The warming process was not unidirectional: two warmer intervals were separated by a slight cooling. A complicated process of climatic fluctuation is reflected in the succession of diatom assemblages, the latter also being influenced by the warm current and the solar heating of coastal water. In the first stage of warming, there existed an oyster reef composed of densely packed shells of Crassostrea gigas. Nineteen associated species were found, 11 percent of them extra-limital warm water species. The presence of these species suggests the summer surface sea water temperature was 6°C-8°C above that of today, on the Tanfiliev coast, at the time of the reef growth. The warming coincided with marine transgression: the latter was once interrupted with a limited regression which coincided with a short period of cooling. The sequence of deposition environments is reconstructed as follows: a shallow bay - a coastal lake - an open bay. The adjacent land was covered with cool-temperate broadleaf and coniferous-broadleaf forests, the coniferous species gaining in importance during cooling. During the second warm phase, the proportion of coniferous and small-leaved species in plant communities was drastically reduced.