Cusptrons produce microwave radiation at the fundamental and harmonic electron cyclotron frequencies by the negative mass instability. A low-energy, axis-rotating beam of 28-30 kV, 0.8-3.5 A, 4 us, and 60 pps is used to generate a single RF-mode both in a circular cavity and in a six-vane circuit. In fundamental and second harmonic frequency generation with a circular circuit, the independently excited modes are TEll's and TE21's with radiation power of more than 1.8 kW and an electronic efficiency of approximately 7.5%. In high-harmonic frequency generation with a six-vane circuit, microwave radiation at 6.0 GHz (sixth harmonic) and 3.9 GHz (fourth harmonic) is also independently generated with more than 10.4 kW and 4.0 kW, respectively. In this case, corresponding electronic efficiencies are approximately 10.0 and 9.5%.