Lacustrine chalk is very common in post-glacial lakes of northern Poland. In the deposit of Lake Kruk lin (NE Poland) carbonates occur as a layer 2-3 m thick. Samples for laboratory studies were collected from three profiles in SW part of the deposit. Mineral composition of the sediments was determined using differential thermal analysis (DT A) and thermogravimetry analysis (TGA), while the age was determined on the basis of pollen analysis. The sediment profiles represent an age of early Holocene. Calcite is the major component of lacustrine chalk. It forms small crystals (<I O μm), sometimes grouped in aggregates, or present as bioclasts. Calcium carbonate precipitated from lake water. Chemical weathering of glacial and fluvioglacial material of the lake catchment is the main source of ions, transported to the lake mainly by groundwater. Formation of the Kruk lin lacustrine chalk deposit must have required 300-350 kg of dry weight calcium carbonate to be carried away from each square meter of the lake catchment. If only top one-meter layer of glacial sediments had been decalcified, it should have contained 13-22 percent of calcium carbonate. The results of petrographic analysis show this value to be possible.