For many years, the primary industrial application for methylotrophs has been the production of single-cell protein, and development in this area continues. Chapter 5 presents a thorough review of this area. Another area, reviewed by Moringa and Hirose in Chapter 4, is the production of secondary metabolites, particularly amino acids. Other areas of application for methylotrophs have been suggested, unfortunately, with no follow-up studies. Silverman 1 suggested the application of methane-oxidizing bacteria to the walls or roofs or coal mines for removal of methane. The analyses of methanotrophs in oil-bearing soil have been suggested for geomicrobiological prospecting of petroleum. 23 However, methane-oxidizing bacteria are widespread in nature and methane measured in soils that are not associated with petroleum is in the order of 5 to 15 ppm. Thus, the presence of methane-oxidizing bacteria is not always indicative of the presence of petroleum or natural gas. 4 A microbial fuel cell has also been suggested, using methy1otrophs and substrate as an anode, and a molecular oxygen electrode as a cathode. 1