Abstract: Continuous control of biosynthesis process conditions on the basis of measurable quantities in the off-gases from the process and calculated quantities derived from said measured quantities provides for optimization of reaction conditions with respect to maximum output rate of selected product. Where the selected product is a mix of cell mass and ethyl alcohol ranging from virtually all cell mass with negligible alcohol to the maximum alcohol content which the cell can tolerate, optimization depends on maintaining the ratio of carbon to nitrogen in the nutrient feed at a level appropriate to the desired product mix. The calculated quantities, including the time derivative of the rate of oxygen uptake, are used for essentially continuous control of the rates of feed of individual nutrients and other selected essential variables as a means of optimizing the process.