A total of 320 potable water samples were collected from various rural and urban sources located in the Lublin region of eastern Poland. They comprised: 55 samples of treated (chlorinated) tap water from rural dwellings distributed by the municipal water supply system (MWSS), 111 samples of treated tap water from urban dwellings distributed by the MWSS, 45 samples of untreated well water from household wells and 109 samples from private water supply systems (PWSS) distributing untreated well water. Water samples were examined for the presence and species composition of Legionella, Yersinia, Gram-negative bacteria belonging to family Enterobacteriaceae (GNB-E) and Gram-negative bacteria not belonging to the family Enterobacteriaceae (GNB-NE), by filtering through cellulose filters and culture on respectively GVPC, CIN, EMB and tryptic soya agar media. The occurrence of Legionella in the samples taken from the outlets of the urban MWSS was high (77.5%), and significantly greater compared to frequencies noted in rural MWSS (7.3%), and samples of well water from household wells (28.9%) and PWSS (13.8%) (p<0.001). Strains L. pneumophila serogroups 2-14, L. pneumophila serogroup 1 and Legionella spp. (species other than L. pneumophila) formed respectively 64.3%, 17.5%, and 18.2% of total isolates from urban MWSS, 100%, 0, and 0 of those from rural MWSS, 69.2%, 7.7%, and 23.1% of those from household wells, and 66.7%, 0, and 33.3% of those from PWSS. The concentration of Legionella strains in the positive samples from urban MWSS exceeded the threshold limit value of 100 cfu/100 ml in 86.1%, while in the other sources this value was not exceeded. No Yersinia strains were isolated from the examined water samples. Altogether 8 species or genera of Gram-negative bacteria belonging to Enterobacteriaceae family (GNB-E) and 10 species or genera of Gram-negative bacteria not belonging to the Enterobacteriaceae family (GNB-NE) were found in the examined samples. In the MWSS samples, an inverse relationship was found between Legionella and GNB-E and the numbers of Enterobacter spp. and Serratia spp. strains were signifi cantly more common in the samples without Legionella. By contrast, in the PWSS samples, the numbers of Enterobacter spp., Klebsiella spp. and Salmonella spp. were distinctly and significantly greater (p<0.01-p<0.001) in the samples containing Legionella. Among GNB-NE, Pseudomonas aeruginosa strains occurred significantly more frequently in samples containing Legionella (for MWSS and well water separately p<0.05, for total samples p<0.001). Similarly, strains of Flavobacterium breve and Xanthomonas spp. occurred significantly more often in the samples with Legionella, while the numbers of Aeromonas spp. and Vibrio spp. strains were significantly greater in the samples not containing Legionella. In conclusion, a health risk could be associated with exposure to the water from urban MWSS because of the high prevalence and concentration of Legionella, and with exposure to well water from PWSS because of the correlation of occurrence of Legionella and potentially pathogenic Enterobacteriaceae strains, and the possibility of synergistic effects. The adverse effects could be also due to the significant correlation of Legionella and Pseudomonas aeruginosa that occured in water from various sources.