Shrimp, crayfish and lobsters possess a reflection superposition compound eye, which operates by reflective optics and is suited to vision in dim-light conditions. The reflector units in the eye are made of millions of crystals of isoxanthopterin, a previously unknown biocrystal. We report here the determination of the crystal structure of isoxanthopterin using first principles calculations based on dispersion-inclusive density functional theory. In a manner akin to the generation of the H-bonded layer and 3D arrangement of the biogenic photonic crystals of guanine, we constructed an H-bonded planar layer of isoxanthopterin molecules, followed by generation of various interlayer motifs, via monoclinic and orthorhombic symmetries. Geometry optimization yielded an isoxanthopterin crystal structure with orthorhombic Cmce symmetry, which gave an excellent fit to the experimental X-ray measurements. The resulting refractive index, calculated from the structure, is very high (n=2.0), explaining why this material functions as an efficient reflector.