The close relation between metabolic activity and heat release means that calorimetry can be successfully applied for on-line monitoring of biological processes. Since the use of available calorimeters in biotechnology is difficult because of technical limitations, a new sensitive heat-flux calorimeter working as a laboratory fermenter was developed and tested for different aerobic and anaerobic fermentations with Saccharomyces cerevisiae and Zymomonas mobilis. The aim of the experiments was to demonstrate the abilities of the method for biotechnological purposes. Fermentations as well as the corresponding heat, substrate and product analyses were reproducible. During experiments the heat signal was used as a sensitive and fast indicator for the response of the organisms to changing conditions. One topic was the monitoring of diauxic growth phenomena during batch fermentations, which may affect process productivity. S. cerevisiae was used as the test organism and a protease-excreting Bacillus licheniformis strain as an industrial production system. Other experiments focused on heat measurements in continuous culture under substrate-limiting conditions in order to analyse bacterial nutrient requirements. Again, Z. mobilis was used as the test organism. Ammonium, phosphate, magnesium, biotin and panthothenate, as important substrate compounds, were varied. The results indicate that these nutrients are required in lower amounts for growth than formerly suggested. Thus, a combination of heat measurements and other methods may rapidly improve our knowledge of nutrient requirements even for a well-known microorganism like Z. mobilis.