A feeding strategy was developed for controlling the specific growth rate in a Bacillus subtilis fed-batch operation, and the importance of the specific growth rate was investigated for subtilisin expression. The ratio of the glucose and peptone concentrations in the feeding medium was critical in controlling the specific growth rate, and the appropriate ratio was found to be 1:2. The specific growth rate was well controlled in fed-batch operations including an exponential feeding of a 200 g/liter glucose and 400 g/liter peptone solution. Controlling the value of the specific growth rate had a significant effect on the subtilisin expression, even though the subtilisin expression was only enhanced after the feeding ended. This would appear to be due to cell components, such as sigma factors, produced in the growth phase that influenced the aprE gene expression. The optimal specific growth rate for the maximum expression rate and longest production period was 0.35 h(-1).