The microbial content of printing paper machines, running at a temperature of 45-50 degrees C and at pH 4.5-5, was studied. Bacteria were prevalent colonizers of the machine wet end and the raw materials. A total of 390 strains of aerobic bacteria were isolated and 86% of these were identified to genus and species by biochemical, chemotaxonomic and phylogenetic methods. The most common bacteria found at the machine wet end were Bacillus coagulans and other Bacillus species, Burkholderia cepacia, Ralstonia pickettii, and in pink slimes, accumulating in the wire area and press section, species of Deinococcus, aureobacterium and Brevibacterium. Paper-making chemicals also contained species of Aureobacterium, B. cereus, B. licheniformis, B. sphaericus, Bordetella, Hydrogenophaga, Klebsiella pneumoniae, Pantoea agglomerans, Pseudomonas stutzeri, Staphylococcus and sometimes other enteric bacteria, but these did not colonize the process water. Yeasts and moulds were not present in significant numbers. A total of 131 strains were tested for their potential to degrade paper-making raw materials; 91 strains were found to have degradative activity, mainly species of Burkholderia and Ralstonia, Sphingomonas and Bacillus, and enterobacteria produced enzymes which degraded paper-making chemicals. Stainless steel adhering strains occurred in slimes and wire water and were identified as Burkholderia cepacia, B. coagulans and Deinococcus geothermalis. Coloured slimes were formed on the machine by species of Deinococcus, Acinetobacter and Methylobacterium (pink), Aureobacterium, Pantoea and Ralstonia (yellowish) and Microbulbifer-related strains (brown). The impact of the strains and species found in the printing paper machine community on the technical quality of paper, machine operation, and as a potential biohazard (Hazard Group 2 bacteria), is discussed.