The gene encoding an FMN-dependent NADH azoreductase, AzrG, from thermophilic Geobacillus stearothermophilus was cloned and functionally expressed in recombinant Escherichia coli. Purified recombinant AzrG is a homodimer of 23 kDa and bore FMN as a flavin cofactor. The optimal temperature of AzrG was 85 A degrees C for the degradation of Methyl Red (MR). AzrG remained active for 1 h at 65 A degrees C and for 1 month at 30 A degrees C, demonstrating both superior thermostability and long-term stability of the enzyme. AzrG efficiently decolorized MR, Ethyl Red at 30 A degrees C. Furthermore, the enzyme exhibited a wide-range of degrading activity towards several tenacious azo dyes, such as Acid Red 88, Orange I, and Congo Red. These results suggested the sustainable utilization of G. stearothermophilus as an azo-degrading strain for AzrG carrying whole-cell wastewater treatments for azo pollutants under high temperature conditions.