The Co beta-cyano derivative of the cobamide isolated from Salmonella typhimurium was identified as Co alpha-(alpha-5,6-dimethylbenzimidazolyl)-Co beta-cyanocobamide, indicating that this bacterium synthesizes 5,6-dimethylbenzimidazole (DMB) de novo. We found that mutants deficient in the synthesis of DMB can incorporate benzimidazole without modification to form Co alpha-(alpha-benzimidazolyl)cobamide, a cobamide that is physiologically active. The analysis of the nutritional requirements of mutants deficient in DMB synthesis identified 4,5-dimethylphenylenediamine as a putative intermediate in the synthesis of the imidazole ring of DMB. Our results suggest that the CobII region of the cob operon of S. typhimurium only encodes functions involved in the synthesis of the imidazole ring of DMB.