Methanotrophs are microorganisms that possess the unique ability to utilize methane as their sole source of carbon and energy. A novel culture system, known as the compulsory circulation diffusion system, was developed for rapid growth of methanotrophic bacteria. Methanol dehydrogenase (MDH, EC 1.1.99.8) from Methylomicrobium sp. HG-1, which belongs to the type I group of methanotrophic bacteria, can catalyze the oxidation of methanol directly into formaldehyde. This enzyme was purified 8-fold to electrophoretic homogeneity by means of a 4 step procedure and was found in the soluble fraction. The relative molecular weight of the native enzyme was estimated by gel filtration to be 120 kDa. The enzyme consisted of two identical dimers which, in turn, consisted of large and small subunits in an ${alpha}_2{eta}_2$ conformation. The isoelectric point was 5.4. The enzymatic activity of purified MDH was optimum at pH 9.0 and $60^{circ}C$, and remained stable at that temperature for 20 min. MDH was able to oxidize primary alcohols from methanol to octanol and formaldehyde.