A calorimetry-supported approach to the measurement of small enthalpic differences in microbial metabolism is presented. Anaerobic fermentations with the bacterium Zymomonas mobilis and the yeast Saccharomyces cerevisiae were carried out in a fermenter-calorimeter at a constant dilution rate (D=0.15 h(-1)). The glucose concentration of the reed was increased stepwise from 10 to 80 gl(-1). Salts, vitamins and trace elements were present in excess. Different steady state conditions were analysed. For Z. mobilis, specific production rates for ethanol, carbon dioxide and heat were three times higher than those measured for S. cerevisiae. If the glucose feed concentration exceeded 20 gl(-1) during the S. cerevisiae fermentations, glucose could be measured in the fermenter. Further increasing glucose concentrations of the feed caused higher specific ethanol, carbon dioxide and heat production rates, whereas the biomass yield Y-X/S decreased remarkably. This result can be explained as the Crabtree effect under anaerobic conditions and interpreted as metabolic uncoupling of anabolic biomass formation and catabolic energy substrate consumption. A futile cycle contributing to the heat production is considered to provide S. cerevisiae with a regulatory mechanism in the control of the flux of metabolites through the glycolytic pathway.