The extremely thermophilic ethanol-producing strain A3 was isolated from a hot spring in Iceland, The cells were rod-shaped, motile, and had terminal spores: cells from the mid-to-late exponential growth phase stained gram-variable but had a gram-positive cell wall structure when viewed by transmission electron microscopy. Strain A3 used a number of carbohydrates as carbon sources, including xylan, but did not utilize microcrystalline cellulose. Fermentation end products were ethanol, acetate, lactate, CO2, and H-2. The temperature optimum for growth was between 70 and 75 degrees C, and growth occurred in the range of 50-75 degrees C. The pH range for growth was 4.7-8.8, with an optimum at pH 7.0. Strain A3 was sensitive to tetracycline, chloramphenicol, penicillin G. neomycin, and vancomycin at 100 mg/l but was not sensitive to chloramphenicol and neomycin at 10 mg/l, which indicates that strain A3 belongs to the eubacteria. Addition of 50.66 kPa H-2 or 2% NaCl did not affect growth. The isolate grew in the presence of exogenously added 4% (w/v) ethanol. The G+C ratio was 37 mol%. 16S rDNA studies revealed that strain A3 belongs to the genus Thermoanaerobacter. Genotypic and phenotypic differences between strain A3 and other related species indicate that strain A3 can be assigned to a new species, and the name Thermoanaerobacter mathranii is proposed.