The incidence and concentration of methanobacteria in the human intestinal tract have not been well evaluated. Only recently were such microorganisms found in the oral cavity. We determined the counts of methanogens in the feces of healthy subjects on a Mediterranean diet and evaluated whether individuals with methanobacteria in the intestine also harbor such bacteria in the oral cavity. Methanobacteria were observed in the feces of 18 of the 20 subjects, at a level ranging from 10(3) to 10(10)/g dry wt. Only 45% of the subjects harbored methanobacteria in the plaque and saliva, showing that the presence of methanogens in the oral cavity was not correlated with their presence in the feces. The presence and number of methanobacteria appeared to remain constant after 3 months. A short rod arranged in chains, similar to the Methanobrevibacter smithii which predominates in feces, was the methanogenic form present in the mouth.