The crystal structure of diopside from Twin Lakes, California, has been refined to a weighted residual of 4.2 percent from intensity data collected at 700°C. The results of this study are virtually identical with those of Cameron, Sueno, Prewitt, and Papike (1973) at the same temperature. In addition, strain ellipsoids describing the thermal expansion for this material have been calculated from the unit-cell parameters determined at nine different temperatures. The values given by Cameron et al. (1973) and Deganello (1973) for other diopsides have been used in similar calculations. These results clearly show that the minimum expansion direction is along the shortest Ca–O bond distance rather than parallel to the tetrahedral chain. The direction of maximum expansion is parallel to the b axis.