The mechanism of coordination of organic bases to the cobalt atom of corrin ring was studied in the biosynthesis of corrinoids with the growing as well as resting cells of Propionibacterium arabinosum. 2-Monochloroadenyl cobamide (MCAC) produced was increased with the amounts of 2-monochloroadenine(MCA) added, depressing the formation of Ψ-B_<12> which is a main product of the microorganism without addition of any base, although the excessive MCA resulted in inhibition of MCAC production and in formation of various carboxylic acids of corrinoids. Even if adenine was added in the presence of MCA, no satisfactory restored formation of Ψ-B_<12> could not be obtained. In every case, a small, but approximately a definite amount of cobalamin was recognized. It means that a definite amount of 5,6-dimethylbenzimidazole(DBI) was always synthesized under the conditions employed and competitively incorporated into the corrinoid molecule. Further, when MCA, adenine and DBI were simultaneously added to the medium for biosythesis, a marked evidence was obtained for indicating that DBI was the strongest, MCA the second, and adenine the weakest cordinating bases to Co in the biosynthesis of corrinoids. A very good agreement was obtained between the results with the growing and resting cells.