Growth of Methanobacterium thermoautotrophicum, an anaerobic archaebacterium using methanogenesis as the catabolic pathway, is characterized by large heat prodn. rates, up to 13 W g-1, and low biomass yields, in the order of 0.02 C-mol mol-1 H2 consumed. These values, indicating a possibly \"inefficient\" growth mechanism, warrant a thermodn. anal. to obtain a better understanding of the growth process. The growth-assocd. heat prodn. (DrHX0,min) and the growth-assocd. Gibbs energy dissipation per mol biomass formed (DrGXmin) were -3730 kJ C-mol-1 and -802 kJ C-mol-1, resp. The Gibbs energy change found in this study is indeed unusually high as compared to aerobic methylotrophes, but not untypical for methanogens grown on CO2. It explains the low biomass yield. Based on the information available on the energetic metab. and on an ATP balance, the biomass yield can be predicted to be approx. in the range of the exptl. detd. 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 redn. and correspondingly by unusually large heat prodn. may be called entropy-retarded growth. [on SciFinder (R)]