The cob operon in Salmonella typhimurium encodes 25 proteins involved in the biosynthesis of cobalamin. Expression of the cob operon is negatively feedback regulated by cobalamin via a translational control mechanism. The concentration of cobalamin required to repress cob expression to half-maximal was determined in vivo and in vitro to 0.4 μM and 0.6 μM, respectively. These results suggest that cob expression in wild-type cells is partially repressed by de novo synthesized cobalamin.