The surface of concrete in drinking water reservoirs is frequently covered with a mineral coating. Since the beginning of the 1980's, there has been an increasing number of reports of brown spots within which the coating matrix was weakened. The diameter of the spots ranged from a few millimetres up to 10-20 centimetres. Cleaning measures resulted in removal of the spot material revealing that shallow pits had formed in the coating. The deeper underlying concrete body was usually unaffected. Although the removed material was shown to contain a substantial microbial biomass, there was no indication of an elevated microbial contamination in the actual drinking water as detectable by the German standard drinking water testing method. The microbial biomass from the damaged sites of six reservoirs was quantified using ester-linked phospholipid fatty acids (PLFA). Population densities were shown to range between 5 × 107 and 5 × 108 cells g−1 in samples collected from non-chlorinated reservoirs and 106 cells g−1 in samples from a chlorinated one (assuming 0.5 fM PLFA equivalent per cell). The recovery of PLFA indicated that physiologically active populations were present in all of the spots sampled and an analysis of the PLFA proriles revealed that the microbial community contained a large percentage of gram negative aerobic heterotrophs. Differences were found between the PLFA patterns of samples from different reservoirs. The fact that a consistent PLFA profile was not recovered from each spot sampled indicates the absence of a single dominant organism. Methyl cellulose (MC) was identified as a possible nutrient source based on the successful growth of bacterial strains isolated from the damaged areas on hydrolyzed coating material. How MC may becomes bioavailable in the reservoirs remains still unclear.