The objectives of this research were to study biogenic synthesis of Ag(0) nanoparticles and to determine the effects of various experimental conditions such as silver nitrate concentrations, pHs and temperatures controlling for the optimal biosynthesis of Ag(0) nanoparticles. The metal-reducing bacteria formed 5-15 nm-sized Ag(0) nanoparticles by reduction of 0.5-1 mM silver nitrate under the conditions around 15-25 degrees C and medium pH 7.5-8.5 within 3 days of incubation. These results not only show that the bacteria enzymatically reduced Ag(I), but also offer methods for microbial synthesis of homogeneous silver nanoparticles and silver recovery from the natural environments.