A simple definition of the energetic growth efficiency both in terms of enthalpy and free energy is given and discussed. In cultures growing by respiratory catabolism, these efficiencies may conveniently be obsd. online by isothermal reaction calorimetry. If the fermentative part of catabolism is important, the free energy growth efficiency must be evaluated based on biomass yield measurements. In continuous cultures of 2 yeasts growing by respiration, by pure fermn., and on a mixed catabolism, the efficiency was always 0.5-0.6 despite widely varying growth yields. [on SciFinder (R)]