AbstractA system of automatic control of substrate inflow into an aerated culture of microorganisms which depend on oxygen‐absorption rate (OAR) has been devised and tested. As the control variable, dissolved oxygen concentration (DOC), which shows the equilibrium between OAR and oxygen‐uptake rate in the microbial culture, was chosen. If the equilibrium is disturbed by changes in OAR, then the oxygen‐uptake rate is changed by substrate limitation. The DOC is measured by means of a Clark‐type polarographic electrode, and the signal is used to actuate the substrate inflow valve or pump. The oxygen‐uptake rate changes of microorganisms, after the addition or exhaustion of substrate in the medium, are so rapid that they can be used for this type of control.Fundamental equations were derived and graphical solutions for the control system parameters were suggested for the steady‐state relations between DOC, oxygen‐uptake rate, specific growth rate, substrate concentration,KLa, and concentration of microorganisms. The system is stable in the entire range of the uptake rates up to nearly the maximum attainable in unlimited substrate conditions, and can be operated in batch feed or continuous flow modifications. It was experimentally tested with the yeastSaccharomyces cerevisiae. The complete utilization of aeration‐system capacity of the fermentor was achieved with high yeast yield and no alcohol formation. The quality of the produc