Abstract The addition of H 2S (500 ppm) to the gaseous substrate (H 2 : CO 2 = 80 : 20) and an increase of mineral components could enhance the growth of a thermophilic methanogen, KN-15, which was isolated from a hot spring in Kyushu, Japan. After 28 h of cultivation, the cell concentration reached 19.5 g dry cell/ l and the maximum methane productivity was 12.5 l/ l/h in batch culture. Although productivities of cell and methane increased exponentially up to 1.1 g dry cell/ l/h and 10 l/ l/h, respectively, these became almost constant after the exponential growth phase by a limitation of the mass-transfer capacity of the fermentor. Cell productivity during the gas limiting phase was proportional to the volumetric mass-transfer coefficient ( k L a) of hydrogen. On the contrary, in continuous culture, cell productivity was almost constant giving 1.1 g dry cell/ l/h under a wide range of dilution rates, so that the cell concentration was inversely proportional to the dilution rate. The growth of KN-15 was dependent only on the mass-transfer capacity of the fermentor in both batch and continuous cultures.