Abstract: In the mesophile production of biogas, the temperature of the liquid used during anaerobic, methanogenic, fermentation is raised to the required temperature (about 35.degree. C.) by aerobically prefermenting solid substrate at substantially ambient temperature and of appropriate moisture content to raise its temperature to a temperature greater than that required for the anaerobic, methanogenic, fermentation. Thereafter flooding the fermenting solid substrate with the liquid at substantially ambient temperature to increase the temperature of the liquid by heat exchange, and then removing the liquid and storing it in a heat insulated environment to reduce the moisture content of the solid substrate to a suitable degree. The above steps of aerobic prefermenting and flooding are repeated until the liquid has reached the required temperature, and then leaving the liquid in situ to enable anaerobic methanogenic fermentation to take place.