The oxygen and thermal conditions in Lake Ińsko were examined in years 1999-2000. Lake Ińsko is one of a dynamictic type and its waters are mixed twice a year. This lake has a strong thermal and oxygen stratification. The oxygen conditions in the lake are very good because the total deficiency of oxygen appears only in summer, on the depth below 30 m, and only in the areas where the influence of town of Ińsko is the strongest. During autumn and winter circulation the concentration of oxygen near the bottom reaches 5 mg O2 dm-3. The concentration of oxygen in the water reflects favorable conditions of biomass production, including primary production of phytoplankton in epilimnion of Lake Ińsko. The intensive development of algas has considerably influenced the transparency of water. On the background of vertical thermal and oxygen differences during the summer there were also differences in the reactions of water.