Structure refinement was made for a clinohypersthene C_<.02> Mg_<.51> Fe_<.47> Si_20_6, derived as a host phase from parental pigeonite by exsolution of augite along the metastable extension of the pigeonite solvus into the stability field of hypersthene. The particular characteristics of the structure are discussed, and compared with those of pigeonite and orthohypersthene. The clinohypersthene shows no indication of anti-phase domain structure on the reflections with (h+k) odd, and has six-fold coordination of oxygen around M2 site. Clino- and orthopyroxenes of hypersthene composition have closely similar atomic parameters. Kinetic factors are expected to be strongly responsible for the metastable extension of the pigeonite solvus.