Using the transmission electron microscope, the ultrastructural examination was conducted to detect the presence of bacterial biofilm on the inner surfaces of the tubing in dental unit waterlines (DUWL). Samples for examination were taken from the tubes providing water to high-speed and slow-speed handpieces, and to an air-water syringe before application of a disinfection procedure. The microscopic analysis made it possible to find the biofilm in all the tubes in the dental unit which were not pre-disinfected. In these samples, no significant differences were found between high-speed, slow-speed and air-water lines.