During fermentation, carbon dioxide formation usually takes place. The absorption of carbon dioxide in water is an exothermic reaction; its release from the medium is endothermic. These effects were first analysed in a fermenter-calorimeter using model media and then monitored during anaerobic batch and continuous cultivation of Zymomonas mobilis cells. The heat effects proved to be functions of the carbon dioxide partial pressure, the pH value of the medium, the gas flow rate through the liquid, and the pump rate of the medium through the fermenter (continuous culture). The heat of biological growth is usually diminished by carbon dioxide desorption. The maximum error introduced in the on-line measurement was calculated to be 48% for anaerobic conditions; the average measured was 35%. The worst case calculated for aerobic conditions was 4%. In the case of a pH-controlled fermentation, the neutralization heat from alkaline feeding contributes to the total heat production measured. When the carbon dioxide partial pressure was maintained constant and the volume of the exhaust gas produced and the alkaline consumption were known, monitoring of the small biological heat production rates under anaerobic conditions was possible with reasonable accuracy.