ABSTRACTEndosymbiotic methanogenic bacteria of three species of anaerobic ciliates(Plagiopyla frontata, Metopus conforms, andM. palaeformis)were inactivated with the specific methanogen inhibitor 2‐bromoethanesulfonic acid. the absence of endosymbiont methanogens reduced growth rate and growth yield by about 30% inP. frontataandM. contortus, while no significant change in fitness was observed inM. palaeformis.InPlagiopylathe growth rate constant is not affected by an artificially increased pH2neither in normal nor in methanogen‐free ciliates. the energetic advantage conferred by endosymbiont methanogens inPlagiopylaand inMetopus contortusprobably is due to excretion of organic material from the bacteria at the expense of bacterial reproduction. It is unlikely that the maintenance of a low pH2within the cells due to H2‐consumption by the bacteria is important to the cil