A membrane-less microbial fuel cell (ML-MFC) with the internal resistance of 3.9 MOmega was used to enrich a microbial consortium oxidizing electron donors with concomitant current generation. Within 4 weeks the system generated a stable current of 2 mA. The current yield was less than 10%. Forced aeration to the cathode compartment generated higher current, but the yield was similar. Use of a cathode with a higher affinity for oxygen could improve the current yield. Additions of NaCl or HCl increased the current generation further with the current yield of 15%. Aerobic microbes turned out to be the predominant oxygen consumer at the cathode. Based on these findings suggestions are made for a ML-MFC configuration with better performance. (C) 2003 Elsevier Ltd. All rights reserved.