An account of the operation of a step‐tunable, low‐power, continuous‐wave gyrotron, GYROTRON V, at the fundamental and second harmonic of the electron cyclotron frequency is presented. The design of this gyrotron was especially optimized for the second harmonic. The gyrotron operating frequency at the fundamental can be tuned from 150 to 300 GHz and extended by second‐harmonic operation to above 600 GHz. Under normal operating conditions where the accelerating voltage is 10 kV and the beam current is 50 mA, power levels in excess of 20 W are obtained at the fundamental and hundreds of milliwatts at the second harmonic. The paper includes observations of single mode operation at the second harmonic and mode competition between second harmonic and fundamental.