The crystal nucleation of nitrogen is studied by molecular dynamics. Using the umbrella sampling technique, we are able to determine the free energy barrier of nucleation and the structure of the critical nucleus formed under typical experimental supercoolings. In agreement with Ostwald's 'step rule', nucleation is shown to proceed into an orientationally disordered cubic phase instead of the thermodynamically stable hexagonal beta phase. We also give an account of the system size dependence of the free energy profiles so obtained. (C) 2003 Elsevier Science B.V. All rights reserved.