The crystal structure of calcite III has been deduced from existing high pressure powder X-ray diffraction patterns, based on the assumption that it is a displacive modification of the calcite I structure. The structure is monoclinic with space group C2 and a Z of 6. There are two Ca and two C positions, and five O positions, and atom coordinates have been refined by distance-least-squares methods to give reasonable octahedral coordination for Ca and parallel, planar CO3 groups. Unit cell parameters refined from a published powder diffraction pattern at 4.1 GPa are: a=8.746(8) ¿; b=4.685(5) ¿; c=8.275(8) ¿; and &bgr;=94.4¿. The structure has a calculated density of 2.949 Mg/m3 at 4.1 GPa which is less than that of aragonite at this pressure and consistent with early piston cylinder studies. This implies that calcite III is indeed a metastable intermediary between calcite I and aragonite.¿ 1997 American Geophysical Union