This contribution presents a simple, unstructured math. model describing microbial growth in continuous culture limited by a gaseous substrate. It has been found that the parameters influencing growth are primarily the gas transfer rate and the diln. rate. Also, growth performance in continuous culture limited by a gaseous substrate was found to be substantially different from \"classic\" continuous culture where the limiting substrate is provided by the liq. feed: The differences between both continuous culture systems are discussed. Continuous cultures of Methanobacterium thermoautotrophicum were performed at three different gassing rates. 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, whereas increasing the diln. rate had little effect on the gas conversion rate but resulted in a dramatic decrease of the biomass concn. [on SciFinder (R)]