---

Background and Objectives: Chemical production of vitamin B12 is a complicated process. The purpose of this study, done for the first time in Iran, was to produce vitamin B12 by Propionibacterium freudenreichii and investigate the effect of adding betaine on its yield. Materials and Methods: Propionibacterium freudenreichii was added to a fermention culture medium containing filtrated soaked corn. This was followed by incubation at 30°C and, then, adding betaine at six concentrations (0, 5, 10, 15, 20 and 30 g/l). Separation and purification were done and the presence and the amount of vitamin B12 produced were determined by HPLC. Results: The most effective concentration of betaine for vitamin B12 production (318.33 Pg/ml) was 10 g/l, which had a negative effect on dry weight of the cells (22.37 g/l). The results demonstrated that betaine could greatly stimulate vitamin B12 biosynthesis by Propionibacterium freudenreichii and inhibit cell growth. Conclusion: Based on the findings of this study, betaine added to the culture medium of Propionibacterium freudenreichii at a suitable concentration could increase the yield of vitamin B12, paving the way to a commercial, more economic method for its production. Keywords: Betaine, Fermentation, Propionibacterium freudenreichii, Vitamin B12