AbstractThe effect of oxygen supply onL‐leucine fermentation was investigated employing a leucine‐producing mutant ofBrevibacterium lactofermentum. Since it was not possible to measure oxygen tension below 0.01 atm by a Teflon‐coated oxygen electrode, the degree of satisfaction of the cells' oxygen demand (cells' respiration rate/maximum oxygen demand of cells,rab/KrM) and the redox potential of the culture medium (E.mV) were used as indices to oxygen supply in cultures under low oxygen tension. When the oxygen demand of the cells was satisfied (rab/KrM= 1.0) and theEvalue was between −90 and −110 mV,L‐leucine formation was 26.5 mg‐ml. When the oxygen demand of the cells was not satisfied (rab/KrM= 0.85) and theEvalue was between −200 and −220mV,L‐leucine accumulation was 29.7 mg/ml. When the oxygen supply was extremely limited (rab/KrM= 0.27) and theEvalue was −280 mV.L‐leucine formation was 12.9 mg/ml. A new method which simultaneously measures the redox potential and dissolved oxygen was applied to the determination of the critical dissolved oxygen level for cell respiration (PLcrit), which was too small to be detected by conventional oxygen electrodes. The value ofPLcritof the leucine producer was