The axisymmetric, negative energy branch of the Doppler‐shifted electron cyclotron eigenmode has been excited on an unneutralized, 15 kA, 2.25 MV, relativistic electron beam propagating along a guide magnetic field interior to a vacuum conducting wave guide. Large amplitude waves have been generated by means of a traveling wave interaction with helical slow wave structures. The growth process has been identified as the resonant coupling of the negative energy cyclotron mode with the positive energy helix modes. The large amplitude wave produced was propagated on the beam and then identified to be the desired symmetric, negative energy cyclotron wave by direct measurement of its azimuthal symmetry, wavelength, and phase velocity direction. The on‐axis axial electric field component of the wave was inferred to be in excess of 10 MV/m. Finally, as is necessary for collective ion acceleration by these waves, the cyclotron wave phase velocity has been controlled, in the range 0.20–0.06 times the speed of light, by spatially changing the guide magnetic field strength.