δ18O and δ13C values were examined from Quaternary lake sediments of the Augustovian Interglacial (Tab.l) in northeastern Poland. The results cannot be easily interpreted, because values of δ18O are high (-4 to -6‰) for cool intervals (according to palynological analysis) and quite low (-8 to -10‰) for warmer intervals. Supplementary mollusc, pollen and isotope examination indicated the presence of a considerable re-deposition of older sediments during lower lake levels mostly during the cool intervals. Assumption that allochthonous calcite from Tertiary marine sediments was transported into the lake basin, allows a reasonable interpretation of the isotope curves. Cool intervals indicate more intensive erosion due to less dense vegetation. Allochthonous older calcite (with high values of δ18O and δ13C) therefore was transported into the lake. On the other hand, warm intervals are characterised by reduced redeposition of older sediments, and therefore values of δ18O are more negative. Although the isotope curves do not permit to draw direct conclusions on temperature and climatic changes, they indicate more or loss intensive erosion rates in the area around the lake.