The aim of this study was bacteriological assessment of the dental unitwaterlines (DUWL) biofilm - concentration and composition of the aerobe and facultative anaerobe bacterialmicroflora, and evaluation of the influence of a disinfecting product, Oxygenal 6, on the biofilm composition.Tubing fragments were taken from 25 units twice, before and after disinfection, and bacterial suspensionof the biofilm was obtained from the samples. The bacterial flora was determined with the plate culturemethod. Bacteria were identified with biochemical microtests: API 20E, API 20NE (bioMerieux, France) and GP2 MicroPlate (TM) (BIOLOG, USA). Before disinfection, the following bacteria were identified: Gram-negative bacteria - Ralstonia pickettii, Pseudomonas vesicularis, Sphingomonas paucimobilis, Xanthomonas maltophilia;Gram-positive cocci - Micrococcus luteus, Micrococcus lylae, Staphylococcus cohnii, Staphylococcus lentus,Staphylococcus spp., Streptococcus spp.; Actinomycetes - Streptomyces albus. The prevailing bacteriawere: Ralstonia pickettii (78.62%), found in all the units, and Sphingomonas paucimobilis (20.45%). After DUWL disinfection, Sphingomonas paucimobilis (88.79%) dominated in the biofilm, Staphylococcus spp. -5.61% and Pseudomonas spp. - 3.74% were next most frequently occurring bacteria, and in more than a halfof the biofilm samples 100% reduction of the bacterial microflora occurred. This study confirms effectivenessof Oxygenal 6 in bacterial decontamination of the DUWL biofilm.