In continuous alcoholic fermentation a strong correlation between CO/sub 2/ evolution rate and ethanol production rate at stationary state is observed. Earlier studies have shown that yields could be increased if substrate inhibition is removed. Productivity decreases drastically at high glucose concentrations with increasing ethanol concentration in the culture medium. In order to achieve higher final ethanol concentrations, substrate concentration should be kept under inhibition limits, e.g., under 100 g/L during all the fermentation time. The Fed-batch approach, therefore, seems a good technique to reach higher productivity. Substrate feeding is delicate in fed-batch operation. Step-wise feeding can either cause excess of substrate or substrate limitation. Yeast alcoholic fermentation does not need such sophisticated control as those generally proposed in the literature. In this communication, a simple device is described which was accurate enough to keep substrate concentration in the limits of 20 +/- 2 g/L.