AbstractVarious phenoxazine, phenothiazine, phenazine, indophenol and bipyridilium derivatives were tested for their effectiveness as redox mediators in microbial fuel cells containingAlcaligenes eutrophus, Bacillus subtilis, Escherichia coli, orProteus vulgarisas the active biological agent, and glucose or succinate as the oxidisable substrate. A ferricyanide‐Pt cathode was used. The open‐circuit cell e.m.f.′s increased in the order of increasing negative formal redox potentials at pH 7(E7m) of the redox compounds. Several of the redox agents worked well as mediators, maintaining steady currents over several hours, and thionine was found to be particularly effective in maintaining relatively high cell voltages when current was drawn from the cell. A number of the compounds tested did not function well, either because they were incompletely or slowly reduced by the microorganisms or because of their instability.P. vulgaris, with thionine as mediator and glucose as substrate, showed the best performance in a fuel cell. This system was examined in some detail under various conditions of external load to establish the effects of organism concentration, mediator concentration, and substrate addition. Coulombic outputs from these cells were calculated by integration of the current‐time plots. Coulombic yields of 30–60% were obtained, on the basis of (theoretical) complete oxidation of added substrate to CO2