Continuous phasing is a technique for cell culturing that is able to reveal metabolic responses during the cycle of cell growth and division. This is accomplished by the correspondence between the periodic addition of a limiting nutrient and the time required for the growth of a generation of cells. The utility of the technique was greatly improved by the development of a feedback system to control the period of nutrient addition. The production of the biosurfactant surfactin was studied using Bacillus subtilis ATCC 21332. The feedback control system was effective in optimizing the period of nutrient addition, resulting in an increase in the production rate of surfactin by at least a factor of ten in comparison to batch culture. The inherent stability of the new technique also provided a means for characterizing the dynamic response of microbial cultures to changes in the environment.