Sewage pipelines in Hamburg made of brickwork showed serious deterioration in the gas space above the sewage level after about 90-130 yrs of service. Frequently 1-2 cm of the surface of the bricks was lost due to crack formation to that depth. The mortar was completely missing for an addnl. 2 cm and was easily removable to a depth of 10-12 cm. To elucidate the reasons for this deterioration, a microbiol. inventory was carried out. From the inner wall of four affected pipelines visually damaged bricks, visually sound bricks and, as a ref., visually sound bricks from the permanently flooded bottom were analyzed. Based on these data, the deterioration was attributed to the metabolic activity of bacteria producing sulfuric and nitric acid - thiobacilli and nitrifiers, resp. For one pipeline, the cell counts for sulfate reducing bacteria pos. correlated with the cell counts for thiobacilli in samples with pH ranging from 3.2 to 8.7. This finding may indicate that the formation of sulfuric acid is at least partially dependent on the presence of sulfate reducing bacteria. In the visually deteriorated bricks the biol. processes, except those of Thiobacillus thiooxidans, were inhibited due to acidic conditions. In the visually sound bricks neutral pH-values were measured. The highest cell counts were detected in these samples. Samples from the red. bricks exhibited alk. pH-values and, consequently, they were only sparsely populated by microorganisms. Simultaneous occurrence of thiobacilli and nitrifiers (acid producing bacteria) and of sulfate reducing bacteria in the visually deteriorate and visually sound sites demonstrates the complex interrelations between these organisms. The deterioration of the brickwork resulted from the dissoln. of alk. binding materials by biogenic acids, causing localized weakening of the structure. Addnl., mech. stress because of growth of calcium sulfate crystals in the pores and cavities contributed to the deterioration.