Microbial fuel cells (MFCs) have been used to enrich microbes oxidizing formate with concomitant electricity generation. Medium containing formate was fed continuously to MFCs. MFCs showed approximately 1 mA of current after 4 months of operation. Over 90% of formate supplied was removed in MFCs, while Coulombic efficiency was only 5.3% indicating substantial electron and energy losses rather than electricity generation. Denaturing gradient gel electrophoresis (DGGE) showed that a formate-utilizing acetogenic bacterium (Acetobacterium sp.), an acetate-oxidizing metal reducer (Geobacter sp.), and another formate utilizer (Arcobacter sp.) were mainly detected on the electrode. This result indicates that some formate was consumed by acetogenic bacteria to make acetate, and acetate was used by acetate-utilizing electrochemically active bacteria (EAB) (e.g., Geobacter sp.). Additionally, formate was oxidized by nonelectrochemically active bacteria under microaerobic conditions in the anode compartment of the MFCs.