This work presents the results of research on physico-chemical and microbiological amounts of heterotrophic bacteria, Actinomycetes, filamentous and yeast fungi in underground waters of aquifers of Omulewskie Reservoir in the Mazurian Lake District in 1989-1992/1993. The examination included water from 11 wells (20-64 m) and 3 piezometric bore-holes (6.5-23.5 m) on woodland (control well), managed by individual farmers and on large farms. Colour of water, pH, dry mass, and total hardness of the examined water samples did not exceed the values accepted for natural hydrochemical background. Electrolytic conductivity rarely exceeded 400 μS/cm. The concentration of the main cations and anions classifies the waters of the examined wells into four types: 1. carbonate-calcium-magnesium; 2. carbonate-calcium; 3. carbonate-chloride-calcium and 4. carbonate-calcium-sulphate. The concentration of oxygen was the lowest in the water from wells situated on large farms, the highest on the area of woodland; on the contrary BOD5. Oxidability only in the water of the wells on the woodland area was the same as the values accepted for the natural hydrochemical background. The amount of total nitrogen was the lowest in the water from the wells situated on large farms and it was correlated with respectively low or high amounts of total phosphorus. The amount of iron reached higher values only in the water of some wells on the area of large farms. Heterotrophic bacteria with higher optimal temperatures and oligotrophic bacteria were the most numerous among heterotrophic microorganisms. Actinomycetes and filamentous fungi were rare and in minimal quantities. There were more yeast fungi, especially those which did not produce carotenoid dyes.