AbstractThe economics of yeast production depend heavily upon the cellular yield coefficient on the carbon source and the volumetric productivity of the process. The application of an on‐line computer to maximize these two terms during the fermentation requires a continuous method of measuring cell density and growth rate. U fortunately, a direct sensor for biomass concentration suitable for use in industrial fermentations is not available. Material balancing, with the aid of on‐line computer monitoring, offers an indirect method of measurement. Laboratory results from baker's yeast production in a 14‐liter fermentor (with a PDP‐11/10 computer for on‐line analyses) show this indirect measurement technique to be a viable alternative. From the oxygen uptake and carbon dioxide production data, gas flow rate, and ammonia addition rate, the cell density during the fermentation has been estimated and found to compare well with actual fermentation data.