EN
This work comprises the results of comparative research on the determining the number of heterotrophic bacteria in underground waters of one of the largest Aquifers without isolation from the surface by an impermeable layer on the Mazurian Lake District. Modified Walters and Schwartz's medium and Groundwater Agar prepared on underground water were used for research. The investigations were carried out in the three years 1991-1993. The calculations of heterotrophic bacteria were made after 1, 2, 3, 4, 5 and 6 weeks of sample incubation at temperatures of 4 and 20°C. Maximum numbers of heterotrophic bacteria were regained in both temperatures on Wolters and Schwartz's medium, on both media at 20°C after 4-6 weeks of sample incubation.