The heaviest member of the rare-earth-metal series lutetium has been studied up to a pressure of 163 GPa in a diamond-anvil cell using energy-dispersive x-ray diffraction with a synchrotron source. The pressures were carefully calibrated using copper as an internal x-ray standard. A phase transformation at (Formula presented) is reported from a double-hexagonal-close-packed structure to a distorted fcc structure (Formula presented) with a volume change of 4.9%. Such a first-order transformation is unusual in an element with a filled (Formula presented) shell and may indicate increased hybridization between the localized (Formula presented) shell and the (Formula presented) conduction band at megabar pressures. We present the equation of state data to 163 GPa and attribute high compressibility to an electronic transfer from (Formula presented) band under pressure. © 1998 The American Physical Society.