Since the discovery of long-range antiferromagnetic (AF) order in the parent
compounds of high-transition temperature (high-Tc) copper oxides,1,2 there have
been tremendous efforts to understand the role of magnetism in the
superconducting mechanism because superconductivity occurs when mobile
�electrons� or �holes� are doped into the AF parent compounds. Much like high-Tc copper oxides, superconductivity in the newly discovered the rare-earth (R) ironbased oxide systems [ROFeAs] are derived from either electron3,4,5,6,7 or hole 8 doping of their nonsuperconducting parent compounds. The parent (nonsuperconducting)LaOFeAs material is metallic but shows anomalies near 150 K in both resistivity and dc magnetic susceptibility3. While optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave(SDW)instability that is suppressed by doping electrons to form superconductivity9, there has been no direct evidence of SDW order. Here we use neutron scattering to demonstrate that LaOFeAs undergoes an abrupt structural distortion below ~150 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then followed by the development of long range SDW-type AF order at ~137 K with a small moment but simple magnetic structure9. Doping the system with flourine suppresses both the magnetic order and structural distortion in favor of superconductivity. Therefore, much like high-Tc copper oxides, the superconducting regime in these Fe-based materials occurs in close proximity to a long-range ordered AF ground state.