A new coccobacillary, nonmotile, Gram-positive, methane-producing organism was isolated from human subgingival plaque. Both hydrogen and carbon dioxide were required for growth. No methane was produced from acetate, formate, or methanol. The optimum pH was 6.9-7.4, and the optimum temperature was 36-38 degrees C. Fecal extract was required for growth, and a volatile fatty acid mixture was highly stimulatory. The DNA G+C content was 28 mol%. On the basis of these characteristics, DNA-DNA hybridization studies, and electrophoretic analysis of cellular proteins, the isolate was considered a new species and named Methanobrevibacter oralis.