The results on bottom sediments from LakeOsinovoyelocated in the Imandra depression (Kola region) are presented. The stages and main features of sedimentation in the OsinovoyeLakebasin during the Late Glacial and Holocene were determined according to diatom data, lithology and radiocarbon data. At the initial stage of lake formation sedimentation occurred in the periglacial environment, with the sea water ingression into the Imandra depression happening at the second stage. According to radiocarbon dating, the paleobasin in the Imandra depression got detached from the sea in the first half of the Preboreal. This is when the conditions favorable for the development of the rich and diverse freshwater diatom flora established there. On the basis of lithological and micropaleontological data separation of the Osinovoye proto-lake from the Imandra palaeolake was recorded in the sediment core at a depth of approximately 90-100 cm. Since that time LakeOsinovoyehas been developing as an independent basin.