Methanogenic bacteria occur in many, but not all, free-living obligate anaerobic protozoa. This sort of symbiosis is especially common among anaerobic ciliates but is also found in a few species of amoebae and flagellates. Protozoa harbouring methanogens have a clostridium-type fermentative metabolism with H2 as metabolite, the hydrogen generation taking place in special organelles, so-called hydrogenosomes. The relation between the host cells and their endosymbiotic methanogens is syntrophic hydrogen transfer. By removing the generated H2, the methanogens stimulate host H2 production, thus increasing the energetic yield of the energy metabolism. This sort of symbiosis has evolved independently in many cases and involves representatives of several major groups of methanogenic bacteria. Symbiotic methanogenesis of free-living anaerobic protozoa plays only a modest quantitative role in terms of CH4 production in most habitats.