d-Monellin is a chemically synthesized protein composed of all d-amino acids. It has an amino-acid sequence identical to l-monellin, a natural protein with potent sweetness. Two crystal forms of d-monellin were obtained. Both crystals were grown under conditions similiar to those used to crystallize natural l-monellin. Crystal form I has similar, but not identical, cell parameters to natural l-monellin and diffracts to 2.7 Å resolution. Crystal form II is very different and diffracts to 1.7 Å resolution using synchrotron radiation.