A novel thermostable glucose dehydrogenase was purified from a Gram-negative moderate thermophilic bacterium isolated from soil near a hot spring. The homogeneously purified enzyme sample showed two peaks at the optimum temperature for reaction i.e., at around 45 and 70 degrees C. However after 30 min of incubation at 70 degrees C, the enzyme showed only one peak at 75 degrees C. From the results of native and SDS-PAGE of this enzyme, it was suggested that at temperatures below 45 degrees C, this enzyme was a hetero-oligomeric complex constructed from two distinct peptides with MWs of 67,000 and 43,000, thereby showing that the optimum temperature for reaction was 45 degrees C. incubation at 70 degrees C of this hetero-oligomer dissociated each subunit and resulted in a single peptide enzyme with a MW of 67,000 that showed GDH activity with optimal temperature only at 75 degrees C. This single peptide enzyme retained more than 80% of its initial activity even after 30 min of incubation at 60 degrees C. Therefore, we concluded that this novel enzyme showed a different optimum temperature for reaction according to its quaternary structure.