SummaryDissolved oxygen levels in fermentation systems are affected by ( a ) gas flow rate, ( b ) degree of agitation, and (c) oxygen partial pressure in the gas. Since the first two variables cannot easily be separated, a method for dissolved oxygen control has been devised based on the variation of oxygen partial pressure in the gas. Oxygen and nitrogen are fed to a special mixing valve capable of varying the proportions of the two gases while keeping the total flow rate constant. A dissolved oxygen electrode and control system adjust the valve setting to automatically maintain a preset oxygen level. Oxygen analyzers, monitoring the inlet and outlet gas streams, provide additional information from which oxygen absorption and uptake rates may be determined. The system provides a means for the quantitative and independent study of aeration and agitation effects. In addition, it offers a basis for routine control of oxygen levels in plant fermentation equipment.