Commercial fuel ethanol production facilities were previously shown to have characteristic populations of bacterial contaminants which reduce product yield and are difficult to eradicate. Bacterial contaminants were found, for the first time, to form biofilms under laboratory conditions. Fermentor samples from a commercial fuel ethanol production facility were used to inoculate a biofilm reactor and purified bacterial isolates were identified. Biofilms were composed of many of the same species present in production samples, with lactic acid bacteria predominating.