Growing cells of Propionibacterium shermanii, when supplemented with imidazole, produced a "complete" corrinoid, as reported from a similar (but also cobinamide supplemented) setup by Muller and co-workers (Eberhard, G.; et al. Biol. Chem. Hoppe-Seyler 1988, 369, 1091). The corrinoid isolate from workup with added potassium cyanide was confirmed to consist mainly (84%) of the suggested Cobeta-cyanoimidazolylcobamide (1), the vitamin B-12 analogue, in which the cobalt-coordinating 5,6-dimethylbenzimidazole base of the vitamin is replaced by imidazole. An extensive NMR spectroscopic analysis of aqueous solutions of 1 was carried out : all H-1 and C-13 signals in the NMR spectra of 1 were assigned unambiguously, exploiting two-dimensional gradient enhanced homonuclear and heteronuclear experiments, including a new variant of the two-dimensional H-relayed X,H-Overhauser experiment at natural abundance, called 2D-HSQC-ROESY, which combines the merits of heteronuclear single quantum coherence (HSQC) and rotating frame Overhauser spectroscopy (ROESY). The structural analyses by NMR, UV, and CD spectroscopy of 1 in aqueous solution did not unravel any gross structural differences of the corrin ligand and of the nucleotide loop of 1 and of vitamin B-12 (2). Cryo-temperature single-crystal structure analyses have been carried out for 1 and 2, the latter one to obtain reference data more accurate than the ones available from Hodgkin’s analysis (Brink-Shoemaker, C. et al. Proc. R. Soc. London, A 1964, 278, 1). The imidazolylcobamide 1 crystallized at room temperature from aqueous acetone in the orthorhombic space group P2(1)2(1)2(1), with Z = 4. Diffraction data were collected at 92 K (a = 15.335(16) angstrom, b = 21.974(9) angstrom, c = 25.501(11) angstrom, V = 8705(2) angstrom3), the structure was solved by the Patterson method and refined to a final R value of 0.1017 for 6105 reflections with F(o) > 4sigma(F(o). Vitamin B-12 (2) was crystallized in the coldroom by vapor diffusion of acetone into a solution of 2 in water/ethylene glycol (20%). The crystal structure was determined at 88 K (orthorhombic, P2(1)2(1)2(1), Z = 4, a = 15.838(7) angstrom, b = 21.927(12) angstrom, c = 25.689(13) angstrom, V = 8921(8) angstrom3) and refined to R = 0.0824 for 5638 significant reflections. In 1 the imidazole coordination at cobalt occurs from the lower face intramolecularly and with the "north/south" orientation also found for the benzimidazole base of vitamin B-12 (2). Comparison of the X-ray data of 1 and of 2 indicates a considerably smaller "upward folding" of the corrin ring in 1 (11.3(0.2)degrees) than in 2 (18.0(0.3)degrees). A contribution to the "upward folding" of the corrin ligand of vitamin B-12 accordingly is indicated here to arise from the bulky, Co-coordinated benzimidazole base of the cobalamins, in support of a hypothesis put forth earlier by Lenhert (Lenhert, P. G. Proc. R. Soc. London, A 1968, 303, 45), when describing the structure of coenzyme B-12.