High-pressure energy-dispersive x-ray-diffraction studies were carried out on the group IVa metals hafnium (Hf) to 252 GPa (to a volume compression V/V0 of 0.48) and titanium (Ti) to 87 GPa using a synchrotron source. Two new structural phase transformations between an hcp phase and an phase and between the phase and a bcc phase (isostructural with group V transition metals) were observed during both compression and decompression in Hf. Equilibrium transformation pressures for Hf are 38 8 and 71 1 GPa for the hcp-to- and -to-bcc phase transformations, respectively, at room temperature. In Ti, only the hcp-to- phase transition was observed above 8 GPa, and the phase was stable to 87 GPa. The crystal structure sequences of Ti, Zr, and Hf at ultrahigh pressures are discussed. © 1990 The American Physical Society.