A newly described bacterial isolate, designated strain NS-E, differs from presently known extremely thermophilic bacteria in various characteristics. It is a strictly heterotrophic eubacterium of marine origin and has a temperature range for growth of 50 to 95 degrees C with an optimum at 77 degrees C and a pH of 7.5. Its DNA base composition is 41.3 mol% guanine + cytosine. It is obligately anaerobic, utilizes various sugars as well as yeast extract, and reduces elemental sulfur facultatively to hydrogen sulfide. In 24-h cultures cell densities are up to fourfold higher in the presence than in the absence of elemental sulfur. Sulfide concentrations of 1.0 and 10.0 mM limit growth by 65 and 95%, respectively. Oxygen sensitivity is apparent only at or above that range of temperature at which growth occurs.