To answer the intriguing question whether or not endothermic microbial growth exists, and in particular, to verify Heijnen and van Dijken's prediction (1992), acetotrophic methanogen, Methanosarcina barkeri, has been cultivated in a highly sensitive bench-scale calorimeter (an improved Bio-RC1 reaction calorimeter) in a pH auxostat fashion. A growth yield of 0.043 C-mol-1 has been obtained and a cell d. as high as 3 g L-1 was attained. Heat uptake during growth has indeed been quant. measured with calorimetry, resulting in a heat yield of +145 kJ C-mol-1. Thermodn. of the growth of acetotrophic methanogens was analyzed in detail. The changes in Gibbs energy, enthalpy, and entropy during growth of M. barkeri were compared with some typical aerobic and anaerobic growth processes of different microorganisms on various substrates. In the growth of M. barkeri on acetate, the retarding effect of the pos. enthalpy change on the driving force of growth is overcompensated by the large pos. entropy change, resulting from converting one org. mol. (acetic acid) to two gaseous products, CH4 and CO2. Both the enthalpy and the entropy increases are due partially to the transition of these two products into the gaseous phase. The thermodn. role of this phase transition for the growth process is analyzed. Microbial growth characterized by enthalpy increase and correspondingly by a large increase in entropy may be called enthalpy-retarded growth. [on SciFinder (R)]