A methanogenic bacterium, present in bovine rumen contents at concentrations of approximately 2 x 10(8) cells per ml, has been isolated in pure culture. The organism is a strictly anaerobic, weakly motile, nonsporeforming, gram-negative rod (0.7 mu x 1.5 to 2.0 mu) with rounded ends. There is a single polar flagellum. The organism grows at temperatures between 30 and 45 C, with an optimum at 40 C, and at pH values between 5.9 and 7.7, with optimal growth between pH 6.1 and 6.9. Of the 17 substrates tested, only formate and H(2) plus CO(2) supported growth. An unidentified, heat-stable factor(s) was required by the organism. The factor, which was not one of the common ones, was present in rumen fluid, mixed rumen bacteria, and yeast extract. On the basis of colony morphology, Gram reaction, and motility, the organism is classified as a new species of methanogenic bacterium, and the name Methanobacterium mobilis sp. n. is proposed.