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Rhodamine 6G (3 PM) effectively inhibited dpH-driven ATP synthesis in Methanobacterium thermoaurotrophicum while dpNA-driven ATP synthesis was not affected by it. Rhodamine 6G inhibited Mg"-stimulated ATPase activity of membrane vesicles prepared from these cells but the ATPase catalytic sector detached from the membrane was insensitive to this inhibitor. Methanogenesis-driven ATP synthesis at pH 6.8 of the cells grown in the presence of 50 mM NaCl was inhibited by rhodamine 6G both in the presence of 5 mM and 50 mM NaCl. On the other hand, the methanogenesis-driven ATP synthesis at pH 8.0 of cells grown in the presence of 50 mM NaCl was slightly inhibited by rhodamine 6G in the presence of 5 mM NaCl and was not inhibited at all in the presence of 50 mM NaCI. The growth experiments have shown that cells of Methanobacferium thermoautotrophicum can grow under alkaline conditions even in the presence of rhodamine 6G and of high NaCl concentration when the growth media were inoculated with the cells which had been grown in the presence of 50 mM NaCI. These results indicate that sodium-motive force-driven ATP synthase in Methanobacterium rhermoautotrophicum operates effectively at alkaline conditions and it might be the sole ATP synthesizing system when the proton motive force-supported ATP synthesis is inhibited by rhodamine 6G.