[en] An intense relativistic electron beam has been injected into a cylindrical drift tube containing a counterstreaming electromagnetic wave (f/sub i/ = 9.3 GHz and P/sub i/ = 170 kW). Within a narrow range of axial magnetic field centered at 5 kG, a reflected wave at f/sub s/ approx. 40 GHz was generated by the interaction of the beam with the incident wave. The reflected wave was observed to have a power of several hundred kilowatts (i.e., greater than the power of the incident wave) and a pulse duration on the order of nanoseconds. All observed experimental characteristics (viz. frequency shift, power amplification, pulse duration, and cyclotron resonance) were consistent with a model of reflection from the discontinuity in refractive index that is associated with an electron beam front near cyclotron resonance. This mechanism could be employed in a new class of short-pulse, ultrahigh power, tunable generators at millimeter and submillimeter wavelengths