We report static pressure compression of cerium metal to 208 GPa (volume compression V/V0 = 0.37) in a diamond anvil cell at room temperature. Cerium is unique in the 4f elements because of proximity of the f shell to the Fermi energy and related phase transformations induced by pressure. The energy-dispersive x-ray diffraction studies were carried out on cerium metal to 208 GPa using a synchrotron x-ray source and an internal copper pressure standard. A collapsed body centered tetragonal phase is found to be stable to the highest pressure with axial ratio remarkably constant at 1.680±0.006 in the 90-208 GPa pressure range in excellent agreement with theory. Cerium is thus isostructural and isoelectronic with 5f-band metal thorium at ultrapressures. We present equation of state parameters, which describe the compression of cerium to ultrapressures. © 1999 American Institute of Physics.