The study was carried on Lake Giłwa (100.8 ha, 9.4 m), in the drainage basin of the Giłwa and Pasłęka rivers. The data obtained in the study allowed the authors to classify Lake Giłwa as a water body belonging to the third stability degree according to PATALAS (1960). As evidenced in the study, Lake Giłwa is a highly eutrophic reservoir. The lake waters were characterized by a high content of nutrients, up to 1.40 mg P⋅dm–3 and 12.47 mg N⋅dm–3. The high fertility of the lake was also exhibited by the values of BOD5 reaching 7 mg O2⋅dm–3, chlorophyll a content (73 mg⋅m–3) and low transparency – 0.7 m. In the peak of the summer, the stagnation oxygen profile is represented by a clinogradecurve typical for eutrophic lakes, while carbon dioxide distribution in the water columnis shown by a „reverse” clinograde curve, also typical for fertile reservoirs. The study has revealed that the water in Lake Giłwa is well buffered, as shown by the alkalinity values, 2.5-5.0 mval dm–3. Total hardness of the reservoir water varied from 157.1 to 278.8 mg CaCO3⋅dm–3, which is typical of hard water. The hardness was conditioned mainly by the calcium content. With the River Giłwa, the lake receives wastewater from the wastewater treatment plant in Gietrzwałd, which is manifested, for example, by the high values of electrolytic conductivity (321-476 μS⋅cm–1), indicating the degree of mineral pollution of the lake. Despite the wastewater input, the amount of chlorides is rather low, 20 mg Cl⋅dm–3 at the most.