The genetic algorithm (GA) was applied for on-line determination of the optimal temperature profile for ethanol fermentation. The optimisation was made by maximising the final ethanol concentration with respect to culture temperature. The effectiveness of the algorithm was examined by on-line measurements of ethanol and cell concentrations. Two computers were employed : one for monitoring, control and data processing, the other for the GA computation. A 14% increase in productivity was achieved by this method compared with the conventional constant-temperature fermentation.