Nitrification is a major problem encountered in many drinking water reservoirsand distribution systems. Nitrification episodes are responsible for sudden dropsin disinfectant residuals, high nitrite levels, high heterotrophic plate counts(HPCs), and occasionally, coliform occurrences including the presence ofEscherichia coli. The corrective measures are expensive and laborious and usuallyonly mitigate the problem for a short time, with a return to nitrification withina few weeks or months. To address this problem, a study was undertaken with thegoal of identifying the microbial populations and the physicochemical conditionsthat mediate or interact with nitrification and coliform occurrences in drinkingwater reservoirs. The ultimate outcome of the study is to identify the specificparameter(s) that indicate or facilitate the establishment and subsequent growthof nitrifying and coliform bacteria, and trigger nitrification and coliformepisodes. Liquid and biofilm samples were collected from three finished-waterreservoirs on a monthly basis. For the biofilm samples, coupons made of the samematerial as the reservoirs (concrete or coated steel) were placed at twodifferent depths to evaluate the effects of water level fluctuations. All sampleswere monitored for nitrifying bacteria, coliforms, and E. coli by polymerasechain reaction (PCR) for sensitivity and specificity, and conventional culturingtechniques for enumeration. On selected sample sets, PCR followed by denaturinggradient gel electrophoresis (DGGE) was performed to identify the organismsmediating or interacting with nitrification and coliform occurrences. A varietyof physical and chemical parameters were also measured. Includes 11 references, tables, figures.