Growth of Methanobacterium thermoautotrophicum, an anaerobic archaebacterium using methano- genesis as the catabolic pathway, is characterized by large heat production rates, up to 13 W g -1 , and low biomass yields, in the order of 0.02 C-mol mol -1 H 2 consumed. These values, indicating a possibly "inefficient" growth mechanism, warrant a thermodynamic analysis to obtain a better understanding of the growth process. The growth-associated heat production (โฌ r H X 0,min ) and the growth-associated Gibbs energy dissipation per mol biomass formed (โฌ r G X min ) were -3730 kJ C-mol -1 and -802 kJ C-mol -1 , respectively. The Gibbs energy change found in this study is indeed unusually high as compared to aerobic methylotrophes, but not untypical for methanogens grown on CO 2 . It explains the low biomass yield. Based on the information available on the energetic metabolism and on an ATP balance, the biomass yield can be predicted to be approximately in the range of the experimentally determined value. The fact that the exothermicity exceeds vastly even the Gibbs energy change can be explained by a dramatic entropy decrease of the catabolic reaction. Microbial growth characterized by entropy reduction and correspondingly by unusually large heat production may be called entropy-retarded growth. ยฉ