Methane-oxidizmg bacteria (methanotrophs) are a unique group of bacteria that grow on methane as then sole source of carbon and energy. They can be isolated from a wide variety of marme and freshwater environments, soils, and sediments and appear to be ubiquitous in the natural environment. They have been classtfred, on the basis of chemotaxonomic studies and 16s rrbosomal RNA phylogenetic analyses, mto five genera: Methylococcus , Methylobacter , Methylomonas , Methylosmus , and Methylocystis (1 ,2 ). These five genera fall into two phylogenetically distinct, exclusively methanotrophic groups. Methanotrophs with type I mtracellular membranes include the genera Methylomonas , Methylobacter , and Methylococcus , which are all related to bacterra of the γ-subdivision of the Proteobacteria . Methanotrophs with type II membranes include Methylosinus and Methylocystis , which belong to the α-subdivision of the Proteobacterla (Table 1 ). There has been considerable interest in methanotrophs since it has been recognized that they are a major sink for atmospheric methane In many natural environments, where these aerobic bacteria are exposed to methane, arising from the biological production by methanogens, they are responsible for removal of much of this methane before it escapes to the atmosphere, and are therefore important in the global carbon cycle (see ref. 3 and chapters therein).