Recently, our group discovered the phenomenon that the sedimentation velocity of colloidal particles in water increased drastically when a horizontal electric field of the order of a few V/mmDC was applied. The authors call this the electrically induced rapid separation (ERS) effect. Regarding the mechanism, it is highly plausible that the particles gather each other in the horizontal electric field. In this study, the sedimentation velocity of poly(methyl methacrylate) (PMMA) spheres in water under a vertical electric field of the order of a few V/mmDC was investigated via transmitted-light intensity measurement. When an upward electric field was applied, the sedimentation velocities of the spheres, whose zeta potential was negative, increased due to the electrophoretic migration toward the cell bottom. When a downward electric field was applied, the sedimentation of PMMA spheres was not observed for a prolonged time, and eventually some of the spheres adhered to the upper positive electrode, and the other spheres settled out in the bottom negative electrode. As a result, in the vertical electric field, rapid sedimentation by forming an aggregation of PMMA spheres has not been observed. These results indicate that the electrical double layers around the spheres play a crucial role in the ERS effect. It is clear that the direction of an electric field plays a crucial role in the ERS effect. (C) 2017 Elsevier B.V. All rights reserved.