A PDP 11-03 microcomputer was successfully interfaced to the fermentation system and yeast Saccharomyces cerevisiae cultivation with this computerized system was carried out using the gas analyzers as the primary measuring elements. The necessary equations and computer programs were developed and used in the system operation. The optimal feed-back control policy computerized and stored in the system utilizes these monitored variables in order to generate the output control signals at proper level. The indirectly determined cell-concentration has predicted the direct measurement rather accurately. The profile of the respiratory quotient obtained upon exercising the on-line fed-back control has shown a very smooth and desirable behaviour and the oberall cell yield has been improved quite significantly as compared with the batch system without control.