Stability criteria and pairwise aggregation rates have been calculated for electrophoretic motion of non-Brownian spheres with differing zeta potentials and thin electric double layers. Aggregation rates are obtained by a trajectory analysis, and stability criteria are determined by analyzing the near-contact motion between two particles. The results predict that a given suspension is considerably less stable against aggregation during electrophoresis than during sedimentation, and that the aggregation efficiencies for electrophoresis exceed those for buoyancy-driven motion by an order of magnitude. These results are explained in terms of the relatively weak interaction between well-separated particles during electrophoresis and the electrically driven withdrawal of fluid between closely spaced particles in an electric field. Electrophoretic and buoyancy-driven aggregation rates depend primarily on a dimensionless driving force and the size ratio of the particles; the stability of a colloidal suspension depends on four additional parameters that describe the colloidal forces that act between the particles.