The hexagonal close packed phase of cobalt (hcp-Co) is associated with numerous stacking faults while the face centered cubic phase of cobalt (fcc-Co) has considerably less stacking faults, as shown e.g. in . Two domains of hcp-Co stacked one above the other with a stacking fault in between are usually delimited by a small interface region of a few fcc-Co layers as shown e.g. in [2,3]. The crystal structures of hcp-Co and fcc-Co should be considered together because the crystallites of these two phases are often clustered together in the same grains of cobalt. The gradual ferromagnetic spin reorientation in hcp-Co between 230 $^\circ$C and 330 $^\circ$C reported in  suggests that this phase could not have a hexagonal symmetry. This hypothesis is verified positively by synchrotron radiation and neutron powder diffraction . The crystal structure of the hcp phase of cobalt is described by the monoclinic symmetry with the magnetic space group C2'/m'. In this monoclinic crystal structure the former hexagonal  axis is no longer perpendicular to the hexagonal layers. The monoclinic structure is an approximate description of the multitude of stacking faulted hcp-Co domains coexisting with fcc-Co domains.
 O. S. Edwards and H. Lipson, Proc. R. Soc. Lond. Ser. A-Math. Phys. Sci. 180, 268 (1942).
 O. Blaschko, G. Krexner, J. Pleschiutschnig, G. Ernst, C. Hitzenberger, H. P. Karnthaler, and A. Korner, Phys. Rev. Lett. 60, 2800 (1988).
 E. Bertaut, A. Delapalme, and R. Pauthenet, Solid State Commun. 1, 81 (1963).
 P. Kozlowski, P. Fabrykiewicz, I. Sosnowska, F. Fauth, A. Senyshyn, E. Suard, D. Oleszak and R. Przenioslo - in preperation.