Corrinoids in several diverse species of methanogens were quantified by a bioassay utilizingEscherichia coli 113-3, a corrinoid auxotroph. All five species examined contained >0.65 nmol corrinoid/mg dry cells when grown on H2/CO2 as carbon and energy source. The highest corrinoid levels (4.1 nmol/mg cells) were found inMethanosarcina barkeri grown on methanol. The amount of corrinoids found in this species was dependent on growth conditions, but, regardless of energy source, metabolized levels inMethanosarcina barkeri were higher than those found in theMethanobacterium species examined (M. arbophilicum, M. formicium, M. ruminantium, andM. thermoautotrophicum).