A new isolate of Salmonella, strain MR4, reduced Mn(IV)O-2 at 2.3 mM under aerobic conditions by about 83% over 24 h. Direct contact of cells to MnO2 was not necessary as the cell-free spent medium produced a similar amount of Mn(II). Pyruvate (1.6 mM) and oxalate (0.8 mM) were identified in the culture medium and presumed to have a role in Mn(II) production in this microorganism.