Molecular dynamics simulations of the diffusion of superoxide ion down the active site channel of the enzyme superoxide dismutase were performed with a parallelized version of GROMOS on the Intel iPSC/860. Our model consisted of a spherical assembly of 6968 atoms centered at a copper ion in the enzyme. Trajectory analysis revealed that the anion is directed toward the copper ion through the cooperative motions of several active site residues. Other mechanistic and structural motifs recurring through five full trajectories are examined. In addition to these qualitative results, an upper bound has been calculated for the rate constant for displacement by substrate of the water molecule that is coordinated to the copper. This required an analysis of the dynamics of crossing a free energy barrier that has been characterized in previous work. Strong frictional effects due to Coulombic interactions lead to a rather small rate constant; the transmission coefficient is less than 0.01. The mechanism of the enzyme therefore may involve diffusion of substrate up to the bound water followed by electron transfer mediated by this water, rather than displacement of the water by substrate with subsequent electron transfer.