An audio-frequency power supply for the filament of an ionization gauge is described. The frequency chosen is higher than the natural frequency of oscillation of the filament, hence the gauge may be used in a magnetic field. Stabilization is achieved by comparing the potential at one end of a high-stability resistance through which the emission current is flowing with a reference potential, and controlling the filament current from the difference of the two. Stability over several weeks of continuous working and over a pressure range of 10^-3 to 10^-6 mm Hg is better than 1%.