The heat generated by both batch and continuous cultures of the yeast Kluyveromycs fragilis was studied using a modified bench scale calorimeter. Batch cultures were used to measure the heat dissipation rates and the heat yields during fully aerobic and completely anaerobic growth, whereas continuous cultures also allowed a quant. study of heat dissipation rates during growth on mixed metab. The extent of fermn. vs. respiration could be specified and controlled by O2 limitation. The heat dissipated per unit biomass formed was highest for fully respirative catabolism and fell continuously to a much lower value typical of anaerobic cultures as the catabolism was shifted increasingly to the fermentative mode. The heat generated/mol O2 taken up stayed quite close to the fully aerobic value of 50 kJ/mol even when a sizable fraction of the substrate available to catabolism was fermented. If the fraction of respiration in the metab. is lowered beyond a certain threshold, the ratio of the heat generation to O2 consumption starts to increase dramatically and finally tends to infinity for fully anaerobic growth. All exptl. results were quant. analyzed and explained on the basis of a simple model which formally describes the cultures in terms of 2 parallel chem. reactions. In simple cases, the model enables calcn. of the whole stoichiometry of the culture from a single measured heat yield. [on SciFinder (R)]