This study concerns a molecular dynamics (MD) simulation, using the embedded-atom method (EAM), of the self-diffusion of an interstitial in the bcc metal tungsten (W) at 2000 K. It is found that the interstitial moves only along 111 diagonals and that the switches to other non-parallel directions take place through a two-dimensional process. The 011 dumb-bell is central to this process. Movement along the 111 diagonals takes place through 111 crowdions occupying 2-6 lattice sites. The probabilities of a direction switch and a move are 0.249 and 0.751, respectively. Translating the complicated movement mechanism into the simple picture of interstitial hopping between lattice points, the diffusion velocity is calculated to be 520 m s^-1 , and the activation energy for the interstitial self-diffusion is calculated to be 0.54 eV/interstitial.