AbstractMeasurements of the heat produced by Saccharomyces cerevisiae CBS 426 were used in conjunction with elemental and enthalpic balances to control fed‐batch fermentations. A proportional control using the ratio of carbon dioxide evolution rate to heat production resulted in high biomass yields and minimal ethanol production. For the special case where the carbon source contains no nitrogen, biomass production estimated from heat measurements agreed well with measured values. When the controller gain was set below the maximum specific cellular growth rate, stable control was achieved, even in response to large upsets in feed concentrations.