Over the past seven years, researchers at the Center for Biofilm Engineering have been performing experiments where reactors operated to simulate drinking water distribution systems were inoculated with one or a combination of five coliform bacteria. These organisms (Klebsiella pneumoniae, Klebsiella oxytoca, Enterobacter cloacae, Enterobacter aerogenes, and Escherichia coli) all exhibited the fecal biotype in EC broth at 45 C and were originally isolated from drinking water distribution systems during regrowth events. In all cases, the coliform(s) were required to compete with naturally occurring heterotrophic bacteria in tap water to form biofilms. The coliforms in these reactors typically are at the level of detection and represent a very small proportion of the total heterotrophic population in the biofilms. As a result, statistical analysis of coliform numbers to establish their response to changes in environmental conditions has been difficult. Regardless of the statistical difficulties, it has been possible to obtain a series of observations on coliform persistence that is of interest and benefit to the drinking water industry. Includes 9 references.