The III-V compound GaAs was studied using energy-dispersive x-ray diffraction with a synchro- tron source up to a pressure of 108 GPa. When the pressure was increased to 16.6 GPa, the GaAs sample transformed from the zinc-blende structure to an orthorhombic structure [GaAs(II)], space group Pmm2, consisting of a primitive orthorhombic lattice with a basis of (0,0,0) and (0,(1/2,), where =0.35. Upon a further increase of pressure to 24 1 GPa, GaAs(II) transformed to another orthorhombic structure [GaAs(III)], space group Imm2, consisting of a body-centered orthorhombic lattice with a basis of (0,0,0) and (0, (1/2,"), where " is 0.425 at 28.1 GPa. With increasing pressure, " approached (1/2 and the GaAs(III) structure gradually assumed the symmetry of the simple hexagonal structure. The transition to the simple hexagonal structure [GaAs(IV)] was completed in the vicinity of 60 80 GPa. The structure remains simple hexagonal up to at least 108 GPa, the highest pressure reached in this study. © 1989 The American Physical Society.