The 5f element thorium has been studied in a diamond-anvil cell to pressures of 300 ± GPa (volume fraction V/V0=0.40) at 300 K by energy-dispersive x-ray diffraction using a synchrotron source and platunum pressure standard. Thorium undergoes a phase transition from its ambient fcc phase to a body-centered-tetragonal (bct) phase near 100 GPa. This bct high-pressure phase of thorium is similar to the one found previously in the 4f lanthanide element cerium above 12 GPa. We suggest that the 5f band which lies above the Fermi energy in thorium at ambient conditions is occupied at megabar pressures and significantly contributes to bonding. © 1991 The American Physical Society.