This article presents a simple, unstructured math. model describing microbial growth in continuous culture limited by a gaseous substrate. The model predicts const. gas conversion rates and a decreasing biomass concn. with increasing diln. rate. The parameters influencing growth are primarily the gas transfer rate and the diln. rate. Furthermore, for correct simulation of growth, the influence of gaseous substrate consumption on the effective gas flow through the system has to be taken into account. Continuous cultures of Methanobacterium thermoautotrophicum were performed at three different gassing rates. In addn. to the measurement of the rates of biomass prodn., product formation, and substrate consumption, microbial heat dissipation was assessed using a reaction calorimeter. For the online measurement of the concn. of the growth-limiting substrate, H2, a specially developed probe has been used. Exptl. data from continuous cultures were in good agreement with the model simulations. An increase in gassing rate enhanced gaseous substrate consumption and methane prodn. rates. However, the biomass yield as well as the specific conversion rates remained const., irresp. of the gassing rate. Growth performance in continuous culture limited by a gaseous substrate is substantially different from \"classic\" continuous culture in which the limiting substrate is provided by the liq. feed. The differences between both continuous culture systems are discussed. [on SciFinder (R)]