Investigations concerning the role of distribution system biofilms on water quality were conducted at a drinking water utility in New Jersey. The utility experienced long-term bacteriological problems in the distribution system, while treatment plant effluents were uniformly negative for coliform bacteria. Results of a monitoring program showed increased coliform levels as the water moved from the treatment plant through the distribution system. Increased coliform densities could not be accounted for by growth of the cells in the water column alone. Identification of coliform bacteria showed that species diversity increased as water flowed through the study area. All materials in the distribution system had high densities of heterotrophic plate count bacteria, while high levels of coliforms were detected only in iron tubercles. Coliform bacteria with the same biochemical profile were found both in distribution system biofilms and in the water column. Assimilable organic carbon determinations showed that carbon levels declined as water flowed through the study area. Maintenance of a 1.0-mg/liter free chlorine residual was insufficient to control coliform occurrences. Flushing and pigging the study area was not an effective control for coliform occurrences in that section. Because coliform bacteria growing in distribution system biofilms may mask the presence of indicator organisms resulting from a true breakdown of treatment barriers, the report recommends that efforts continue to find methods to control growth of coliform bacteria in pipeline biofilms. Ample microscopic evidence is available to show that most pipe surfaces in distribution systems are colonized by microorganisms. Allen et al. (1, 2) showed high populations of bacteria in main encrustations collected from water utilities throughout the United States. Diatoms, algae, and filamentous and rod-shaped bacteria were commonly encountered. In contrast, Ridgway and Olson (35) found that bacteria were sparsely distributed along pipe surfaces they examined. These sparse communities, however, showed a variety of morphologically distinguishable bacterial structures including rodand chain-forming cocci and filamentous and prosthecate cell types. Some organisms were attached to pipe surfaces by extracellular fibrillar appendages. Helical stalks, characteristic of the iron bacterium Gallionella, were observed both in water samples and attached to pipe surfaces (35, 36). Scanning electron microscopy showed that 17% of the 10-to 50-p.m-sized particles were colonized with 10 to 100 bacteria per particle (35). Culture examination of distribution system biofilms has demonstrated large variations in the number of heterotrophic plate count (HPC) bacteria. Olson (31) reported Acinetobacter densities on mortar-lined pipe surfaces as high as 109 bacteria per cm2. Tuovinen and Hsu (40) found viable counts ranging between 40 and 3.1 x 108 bacteria per g in tubercles collected from water distribution pipelines in Columbus, Ohio. Tubercles were found to contain several types of...