Theoretically, 18 geometrical isomers can appear,i.e. three groups of six isomers; in each of the six in one group histidine as a bidentate ligand is coordinated either through Ni and N~, Nt and O-or N~ and Orespectively. The second notable feature of the structure of histidine compounds is that the histidine moiety occurs in two conformations, 'open' and 'closed'; the magnitude of the torsion angle about the C~-C ~ bond differentiates between the two conformers. The total number of geometrical isomer-conformers is therefore much larger. The X-ray crystal-structure analysis reported here was undertaken to determine the structural isomerism of the orange isomer of the [CoNO2(L-his)2] complex (Kamberi et al., 1980).