A system is described for the control of the position of the ion beam in a mass spectrometer. The instantaneous position of the beam with respect to the point of maximum collector current is presented on a cathoderay tube indicator. The beam can be held at this maximum point either manually using this indication, or by automatic control circuits. In automatic operation the beam first ``searches'' for the peak by sweeping slowly across the region of the collector plate. It is then ``locked in'' and hunts about the point of maximum current. The amplitude of the hunting can be made sufficiently small so that the average collector current does not differ greatly from the peak current. The beam remains locked in position during slow drifts of magnet current and acceleration voltages. After large sudden fluctuations it returns to search operation and locks in shortly thereafter. The method of control is an adaptation of principles used in automatic frequency control of radar receivers.