Bacteria capable of growth on reduced carbon-one compounds have been commanding the attention of an increasing number of microbial physiologists and biochemists. In addition to the carbon-one substrates, methanol and formic acid, there are a number of naturally occurring compounds which are regarded as methylotrophic substrates since they do not contain carbon-carbon bonds. Methylosinus trichosporium is the only methanotroph in which measurements of respiration-induced proton extrusion have been successfully made. The obligate methane utilizer, Methylococcus capsulatus, when grown on methanol retains the membranes characteristic of growth on methane. A considerable proportion of carbon turnover in the carbon cycle passes via methane produced by methanogenic bacteria, and it is, therefore, not surprising that methanotrophs are widely distributed in the environment. For methane oxidation, addition of nicotinamide adenine dinucleotide as electron donor is required. In this species, the enzymes that oxidize methane to formate are localized in membranes which can account for 40 to 60% of the total mass of the bacterium.