Microbial synthesis of gold nanoparticles was achieved at 25°C and pH 7-1 using the mesophilic bacterium Shewanella algae with H2 as the electron donor. The microbial synthesis of gold nanoparticle was a fast process: 1 mM AuCl4− ions were completely reduced to insoluble gold within 30 min. At the solution pH 7, the gold nanoparticles of 10-20 nm were synthesized in the periplasmic space of S. algae cells. When the solution pH was decreased to 1, the gold nanoparticles of 50-500 nm were precipitated extracellularly. The solution pH was an important factor in controlling the morphology of biogenic gold particles and location of gold deposition.