Glucose dehydrogenase was purified to homogeneity from the thermoacidophilic archaebacterium Thermoplasma acidophilum. The enzyme is a tetramer of polypeptide chain Mr 38,000 +/- 3000, it is catalytically active with both NAD+ and NADP+ cofactors, and it is thermostable and remarkably resistant to a variety of organic solvents. The amino acid composition was determined and compared with those of the glucose dehydrogenases from the archaebacterium Sulfolobus solfataricus and the eubacteria Bacillus subtilis and Bacillus megaterium. The N-terminal amino acid sequence of the Thermoplasma acidophilum enzyme was determined to be: (S/T)-E-Q-K-A-I-V-T-D-A-P-K-G-G-V-K-Y-T-T-I-D-M-P-E.