This work presents the results of research on quantitative and qualitative composition of heterotrophic bacteria in waters from 11 wells and 3 piezometric bore-holes in Omulewski Reservoir Aquifer before liquidation of large cattle and swine farms (in 1989-1991) and after their liquidation (1994-1995). Examinations concerning the number of indicatory heterotrophic microorganisms on TGY medium showed fluctuations in the number (from some to several thousand CFU/1 cm³ of water) depending on the way of utilization of the soil and the research period. Gram-negative rods (44-79%) dominated in 241 water samples taken from wells and piezometric bore-holes in both research periods. Gram-positive coccus (3-32%) and cells rod-shaped (3-19%) were not so numerous. Strains: Achromobacter, Flavobacterium and Xanthomonas dominated among Gram-negative bacteria irrespectively to the research period and a group of wells. Gram-positive bacteria were mainly represented by: Bacillus, Sarcina and Micrococcus.