Abstract An experimental and theoretical research program to study the ubitron-FEL interaction is in progress at the Naval Research Laboratory, including both fundamental and harmonic modes of operation. Operation of a ubitron-FEL amplifier using the fundamental wiggler harmonic interaction with the TE11 circular-waveguide mode is reported here. The experiment employs a 190–250 keV electron beam with currents up to 37 A, using a helical wiggler/axial guide-field configuration. Small-signal gains as high as 17–19 dB have been observed in the 13–18 GHz frequency band, corresponding to a gain per free-space wavelength approaching 1.5 dB/λ. These results were obtained using a modified SLAC klystron gun. Improved performance is expected using a higher-current (100 A), higher-quality electron gun currently being installed. The theoretical program includes a nonlinear analysis of the rf and electron beam interaction while propagating through a cylindrical waveguide with a helical-wiggler field and an axial guide field. The analysis includes beam injection into the wiggler, wiggler inhomogenieties, efficiency enhancement by means of tapered wigglers, beam thermal effects and harmonic interactions. The experimental results presented in this paper are in reasonable agreement with the nonlinear simulation of the interaction in the Raman regime.