A description is given of an experimental investigation of a continuous-wave magnetron oscillator, the magnetic field for which is provided by an electromagnet energized by the anode current of the valve. Oscillations can be readily obtained, and they persist over a wide range of anode voltages. With a two-segment-anode valve oscillations take place at the fundamental frequency of a lecher-wire circuit connected to the valve. With a four-segment-anode valve, oscillations at the fundamental frequency of such a circuit are observed at low anode voltages, but as the voltage is increased oscillations at 3, 5, 7 … times the fundamental frequency appear successively. The range of frequency covered by the experiments was 40–750 Mc/s (λ 7.5 m to 40 cm). During oscillation the anode current assumes the value necessary to provide the optimum magnetic field, and, within limits, the number of turns on the electromagnet and the magnitude of the filament current do not affect the action of the oscillator. The operational stability is good, and the danger from an excessive anode current is largely removed.