This invention relates to an improved anaerobic digestion apparatus for decomposable organic materials characterized by a digestor covered and sealed by a liquid-filled pond heated with solar energy. The effluent from the fermentation reaction taking place in the digestor has excellent absorptivity for solar energy and is advantageously used as the heat transfer medium filling the pond. The pond is covered with a translucent roof capable of transmitting solar energy, such roof cooperating with the liquid-filled pond therebeneath to produce an artificial environment of reduced sensitivity to atmospheric conditions which is operative to help maintain a condition of stable equilibrium within the digestor. In the preferred embodiment, the vessel forming part of the digestor in which the fermentation reaction takes place comprises an excavated trench lined with a suitable fluid-impermeable membrance while the roof over the pond consists of an inflatable air-supported bubble. Also preferred is to float the pond upon the fermenting material in the digestor by covering the latter with a flexible fluid-filled float-supported membrane. The invention also encompasses the novel method of carrying out anaerobic digestion of decomposable organic materials which includes sealing and insulating the open-topped reaction vessel by covering same with a fluid-filled pond, heating the fluid in the pond with solar energy, and circulating the fluid thus heated through the feedstock in the digestor so as to maintain a near constant temperature therein conducive to fermentation. The invention further includes such novel steps as floating the pond upon the fermenting feedstock in the digestor, burying the reaction vessel to minimize heat transfer through the walls thereof, and establishing a self-regulating environment above the digestor which insulates same from the more radical, and sometimes extreme, fluctuations in atmospheric temperature and light conditions occurring outside thereof.