Au-Ag alloys have been synthesized using the fungus Fusarium oxysporum. It has been found that initially Au is reduced followed by Ag leading to nanoparticles of Au-Ag alloy. The gradual change of color from purple to deep brown is the clear indication of the formation of bimetallic Au-Ag alloy nanoparticles. Depending on the biomass of fungus used one can change the composition of the alloy nanoparticle formed from pure Au to pure Ag with increase in the biomass concentration. Exactly the same effect can be achieved at a lower biomass concentration by adding extraneous NADPH to the solution containing the biomass and the salts.