The structure of bis-(L-histidino)cobalt(II) monohydrate, Co(C6H8N3O2)2, H2O, has been determined from three-dimensional X-ray diffraction data. The crystals are monoclinic with a= 29·44, b= 8·324, c= 6·347 Å, β= 90·0°, and space group C2. Positional and anisotropic thermal parameters have been refined by least-squares methods; the final R factor is 0·125. The cobalt atom is octahedrally co-ordinated to the amino-nitrogen, an imidazole nitrogen, and an oxygen atom of each histidine group, at distances between 2·10 and 2·20 Å. The molecule has approximate two-fold symmetry (not cyrstallographic) with the imidazole groups trans to each other.