Starting with monodisperse spherical polystyrene particles ranging in diameter from 208 nm to 3.90 mu m, a series of ellipsoidal particles have been prepared by a previously published stretching technique. Both spherical and ellipsoidal particles were examined by microelectrophoresis. The electrophoretic mobilities so obtained were converted into xi-potential values using the Smoluchowski equation and an equation obtained by O’Brien and Ward from an analysis of the electrophoretic motion of spheroidal particles.