This work describes bacteriological studies on the quantitative and qualitative composition of bacteria from the family Enterobacteriaceae as well as numbers of Pseudomonas aeruginosa and Aeromonas hydrophila in underground waters of the Omulewski Reservoir, a source of water for consumption. In 1994-1995 studies were carried out on 11 deep wells and 3 piezometric holes. Bacteria from the family Enterobacteriaceae were found in amounts from a few to many colonies (cfu/100 ml), depending on the sampling station and the method of land use (groups of wells). In the entire study period and usually in all wells the following dominated: Citrobacter freundii, Ervinia herbicola, Escherichia coli, Proteus vulgaris, and Serratia marcescens. Pseudomonas aeruginosa and Aeromonas hydrophila were present in small amounts in the majority of water samples (from a few to several colonies) and only when large quantities of water were collected (100 ml).