Highly dispersed palladium nanoparticles supported on bacterial cells were successfully prepared by a microbial method using the metal ion-reducing bacterium Shewanella oneidensis. Resting cells of S. oneidensis reduced soluble palladium(II) to insoluble palladium(0) at room temperature and neutral pH within 60 min when formate was provided as the electron donor. Transmission electron microscopy analysis of a thin section of S. oneidensis cells after exposure to a PdCl2 solution revealed that palladium particles approximately 510 nm in size were deposited on the bacterial surface and in the periplasmic space. The initial concentrations of soluble palladium(II) and formate in the precursor solution strongly influenced the rate of palladium(II) reduction and the dispersity of biomass-supported palladium particles. The dried biomass-supported palladium was tested as an anode catalyst in a polymer electric membrane fuel cell for power production. The maximum power generation of the highly dispersed biomass-supported palladium particles was comparable to that of a commercial palladium catalyst. (C) 2010 Elsevier B.V. All rights reserved.