The corrinoids synthesized by the sulfate-reducing bacterium Desulfovibrio vulgaris were analyzed. The compounds found were guanylcobamide and hypoxanthylcobamide; structures were determined by mass spectrometry, 1H-NMR, and ultraviolet/visible spectroscopy. D. vulgaris used externally added guanine to form guanylcobamide, as demonstrated with 8-14C-guanine. Addition of adenine did not lead to the formation of adenylcobamide (pseudovitamin B12), whereas 5,6-dimethylbenzimidazole was transformed into vitamin B12.