Subfossil diatoms analysis was employed to reconstruct past environmental changes in Lake Skaliska. This lake, presently a palaeolake, is located on a wide plain called the Skaliska Basin (northern part of Mazury Lake District, north-eastern Poland). Changes in terrestrial vegetation suggest that the initial phase of the lake was in the early Holocene. In the sediments a total of 176 diatom species belonging to 35 genera were identifi ed. The majority of diatoms are alkaliphilous and alkalibiontic, occurring mainly in meso-eutrophic water. Diatom fl ora development suggests that the best conditions for diatom growth prevailed throughout the Boreal and in the early Atlantic, a suggestion supported by the increased frequency of planktonic diatoms living in nutrient-rich water. A water pH reconstruction (DIpH) based on diatoms points to alkalinity during the lake’s existence. Since roughly the mid-Atlantic the lake was shallowing, and at the beginning of the Subboreal peat sedimentation led to complete overgrowth of the lake.