The crystal structure of the title compound, Ca(C6H2N3O7)2, 5H2O, has been determined by single-crystal X-ray diffraction at 295(1) K and refined by least squares to a residual of 0.049 for 1513 'observed' reflections. Crystals are orthorhombic, Pmab, a 24.169(6), b 10.292(7), c 8. 554(2) Å, Z4. The stereochemistry about the calcium has not been observed previously for the system [M(bidentate)2-(unidentate), ]; in the present structure, the calcium is coordinated by a pair of bidentate picrate ligands and the four water molecules in an array in which three of the water molecules occupy a triangular face of a square antiprism, the overall array having m symmetry. The remaining water molecule occupies a lattice site with no close interaction with the other species.