Volatile sulfur compounds (VSCs), including hydrogen sulfide, methyl mercaptan, and dimethyl sulfide, are primarily responsible for oral malodor. Recently, the mgl gene encoding L-methionine-alpha-deamino-gamma-mercaptomethane-lyase, which produces methyl mercaptan, was cloned from Porphyromonas gingivalis. This article discusses the mechanism and pathogenic role of the formation of VSCs by oral bacteria.