Methanogenesis per ciliate protozoan and most probable numbers (MPN) of methanogens per ciliate were measured and observed to be affected by feeding. The close similarity observed between the changes in apparent methane production and the changes in the number of methanogens per ciliate indicated that apparent methane production by ciliates depended primarily on the number of methanogens associated with them. Large changes in the MPN of methanogens associated with ciliates occurred rapidly. It is likely that these changes reflect the number of methanogens that associated extracellularly with the ciliates. Methanogens associated with ciliates may have differed from the free living methanogens in halophilism and their resistance to ammonium ions.