Gate valves with rubber-coated wedges were found to be the cause of coliform contaminations of drinking water without disinfectant residual. The characteristic organism was Citrobacter freundii, which was isolated from the drinking water as well as from the surface of the rubber-coated wedges. The rubber on the wedges was covered with excessive biofilms in which the coliform bacteria must have multiplied after initial inoculation by contaminated water. Thus, one of the prerequisites for minimizing the potential of coliform proliferation was to reduce biofilm formation on the rubber surface. This was achieved by developing new rubber types with strongly reduced proportions of microbiologically degradable components. In drinking water distribution systems rubber materials with excessive biofilms provide a potential habitat for potentially pathogenic bacteria and have to be considered as a potential source of contamination in non-chlorinated drinking water. Includes 10 references, tables, figures.