The first observation of microwave generation at the sixth harmonic of the electron cyclotron frequency using a 17 kV, 0.6–1.0 A, 3 &mgr;sec electron beam is reported. This device, which is called a cusptron, produces radiation from the resonant interaction of a nonrelativistic rotating electron beam and a magnetron‐type conducting boundary with six vanes by the negative mass instability. The observed radiated power is approximately 500 W at 4.38 GHz.