The crystal structure of LiBH4 has been studied by synchrotron X-ray powder diffraction at room temperature and at 408 K. At room temperature it has orthorhombic symmetry [space group Pnma, a=7.17858(4), b=4.43686(2), c=6.80321(4) Å]. The tetrahedral (BH4)− anions (point symmetry m) are aligned along two orthogonal directions and are strongly distorted with respect to bond lengths [B–H=1.04(2)–1.28(1) Å] and bond angles [H–B–H=85.1(9)°–120.1(9)°]. As the temperature is increased the structure undergoes a first-order transition and becomes hexagonal (space group P63mc, a=4.27631(5), c=6.94844(8) Å at T=408 K). The (BH4)− tetrahedra align along c, become more symmetric [point symmetry 3m, B–H=1.27(2)–1.29(2) Å, H–B–H=106.4(2)°–112.4(9)°] and show displacement amplitudes that are consistent with dynamical disorder about their trigonal axis