The room-temperature equation of state of silicon, as determined from in situ energy-dispersive x-ray diffraction using a synchrotron source, is presented to 248 GPa. An intermediate phase between the primitive hexagonal and hexagonal-close-packed (hcp) phases is stable from 37.6 1.6 to 41.8 0.5 GPa, and is shown to be the same structure as the X phase of the alloy Bi0.8Pb0.2, and is not a simple restacking of hexagonal-close-packed layers (i.e., Sm-type, dhcp, or thcp). The hcp fcc phase transition occurs at 792 GPa. The fcc phase remains stable to 248 GPa, where the silicon fractional volume is V/V0=0.361 0.006. Excellent agreement between first-principles total-energy calculations and these results is observed for the hcp fcc transition pressure, and the fcc Si pressure-versus-volume equation of state. © 1990 The American Physical Society.