THE behaviour of molecular hydrogen at high pressures has implications for the interiors of the giant planets, which consist mainly of hydrogen, In particular, the question of whether solid hydrogen becomes metallic under these conditions has been much debated(1-9), in part because the structure that molecular hydrogen adopts at high pressure is not known, Here we report the results of first-principles molecular dynamics simulations of solid hydrogen at pressures up to 270 GPa. We find that at 77 K, hydrogen exists as a stable, orientationally disordered phase up to 60 GPa, consistent with experimental results(1,10). As the presssure is raised, a gradual transformation to an ordered orthorhombic structure begins at 160 GPa, and by 260 GPa the solid becomes semiconducting, with an indirect band gap of 1.4 eV. The calculated vibrational density of states of this phase is consistent with infrared and Raman spectra measured up to 160 GPa (ref. 11). Although limitations on the simulation time and size may result in an overestimate of the absolute pressure, our calculations show that solid hydrogen does not become metallic, even at pressures approaching 260 GPa.