Bi21.2(1)(Mn1-xCox)20 is a new metastable phase which is synthesized via Bi self-flux, adopts a highly fibrous morphology, and decomposes endothermically near 168 °C. It crys-tallizes in the orthorhombic space group Imma with unit cell parameters a = 19.067(4) Å, b = 4.6071(10) Å and c = 11.583(4) Å, adopting a low-temperature modification of BiNi-type struc-ture by forming columns along the b-axis. Wavelength-dispersive X-ray spectroscopy (WDS) confirms the presence of Co in the structure, which is found to be 7 at.%. In each column, the transition metal (T) and Bi atoms construct a double-walled nanotubular arrangement of atoms around the disordered central Bi atoms. Electronic structure calculations (LMTO-ASA, LSDA) show that the calculated Fermi level falls into a pseudogap and also indicate a possible low-temperature magnetic ordering in the phase.