The aim of this study was bacteriological assessment of water in dental unit reservoirs--concentration and composition of the aerobe and facultative anaerobe bacterial microflora. Reservoir water samples were taken from 25 units. Bacterial flora were determined with the plate culture method. Bacteria were identified with biochemical microtests: API 20E, API 20NE (bioMérieux, France) and GP2 MicroPlateTM (BIOLOG, USA). The concentration of total bacteria isolated from one site was 201,039 cfu/ml, on average; the minimum was 22,300 cfu/ml, and the maximum - 583,000 cfu/ml. The following bacteria were identified: Gram-negative bacteria--Brevundimonas vesicularis, Moraxella lacunata, Moraxella spp., Ralstonia pickettii, Sphingomonas paucimobilis, Stenotrophomonas maltophilia; Gram-positive cocci--Micrococcus luteus, Micrococcus lylae, Staphylococcus cohnii, Staphylococcus hominis ss novobiosepticus, Staphylococcus spp., Streptococcus spp.; actinomycetes--Streptomyces albus. The prevailing bacteria were: Ralstonia pickettii (96.46%), found in all the units. Sphingomonas paucimobilis (1.32%) and Brevundimonas vesicularis (1.07%) were the next most frequently occurring bacteria. Bacteria concentration in dental unit reservoirs reached excessive values, and the bacterial flora were composed of the bacteria characteristic for water supply systems, opportunistic pathogens, and bacteria of the oral cavity flora. Continuous microbiological monitoring of the DUWL water, including application of a disinfecting procedure, is necessary.