The stability and pairwise aggregation rates of small spherical particles in a heterogeneous suspension under the collective effects of gravitational motion and electrophoretic migration are analyzed. The particles are assumed to be non-Brownian, with thin, unpolarized double layers and different zeta potentials. The gravity vector and the electric field are assumed to be oriented in either the same direction or opposite directions. The particle aggregation rates are always enhanced by the presence of an electric field for parallel alignment of the gravitational and electrophoretic velocities. For antiparallel alignment with the magnitude of the gravitational relative velocity exceeding the magnitude of the electrophoretic relative velocity between two widely separated particles, the particle aggregation rates are reduced by the presence of the electric field, and there is a "collision-forbidden" region in parameter space due to stronger hydrodynamic interactions of the particles for gravitational motion than for electrophoretic motion. For antiparallel alignment with the magnitude of the electrophoretic relative velocity exceeding the magnitude of the gravitational relative velocity between two widely separated particles, the particle aggregation rates are enhanced by the presence of the electric field.