Some bacteria have a higher tendency to produce biofilm than others. Especially, Pseudomonas and Aeromonas strains are acknowledged to be pioneer colonizers and are predominant in biofilm formation. We examined biofilm formation and first attachment maintance of biofilms of Pseudomonas spp., Pseudomonas aeruginosa, Aeromonas spp, sulphate reducing bacteria and filamentous fungi. A simulated recirculating cooling-water system was used. Heterotrophic bacteria counts on galvanized steel and glass surfaces rose during the tidy period of 720 hours. In addition, we determined that although Pseudomonas spp., Pseudomonas aeruginosa and Aeromonas spp. were the pioneer colonizers, they surprisingly could not be determined in the biofilms on both types of surface after 456 hours. Sulphate reducing bacteria were observed in biofilms on both surfaces from the outset of the experiments. Filamentous fungi were seen on the galvanized steel and glass surfaces after 0.5 h.