Unicarbon substrate mixtures (methanol/formate/CO2) have been used to formulate growth conditions in which both growth and the intracellular concentration of vitamin B12 have been improved. This approach has enabled the specific concentration of B12 to be increased (x 2.5) without diminishing production rates. Levels of B12 was shown to be directly proportional to the metabolic flux passing through the cobalamin dependent methyltransferase reaction necessary for methylotrophic growth.