Based on principles which are outlined, an apparatus for alternating fields microelectrophoroesis is described. It uses a laternal orientation cell which is moved vertically at constant speed while a wave burst is applied to the cell. The resulting particles movements are photographed through dark‐field illumination and a microscope. The mobility is deduced from the track amplitudes on the photographs. An easy temperature correction is given. The main advantage of the A. C. microelectroesis method is a higher precision in the determination of the mobility distribution curve.