A methane-oxidizing bacterium was isolated from the effluent of manure and its molecular and biochemical properties were characterized. The isolate was aerobic, Gram-negative, and non-motile. The organism had a type I intracytoplasmic membrane structure and granular inclusion bodies. The outer cell wall surface (S-layers) was tightly packed with cup-shaped structures. Colonies were light yellow on nitrate mineral salt agar medium. In addition, the organism was catalase and oxidase positive. The isolate used the ribulose monophosphate (RuMP) pathway for carbon assimilation, and was able to utilize methane and methanol as a sole carbon and energy source, however, it could not utilize any other organic compounds that were tested. The cells grew well in a mixture of methane and air (methane:air=1:1, v/v) in a compulsory circulation diffusion system, and when grown under those conditions, the optimum pH was approximately 7.0 and the optimal temperature was $30^{circ}C$. In addition, the specific growth rate and generation time were 0.13 per h and 5.43 h, respectively, when grown under the optimum conditions. The major ubiquinone was Q-8, and the G+C mol% of the DNA was 55.3. Phylogenetic analyses based on the 16S rRNA gene sequence comparisons showed that this bacterium belongs to a group of type I methanotrophs, and that it is most closely related to Methylomicrobium, with a sequence similarity of 99%. Therefore, the isolate was named Methylomicrobium sp. HG-1.