A free electron laser (FEL) oscillator experiment is described, in which an intense electron beam (750 kV, 20 kA) is prepared in a region of increasing guide magnetic field before injection into an undulator (60 periods,

cm). The electron motion accordingly has a substantial transverse component (

), and motion at the cyclotron (

) and undulator (

) frequencies occurs. Theory predicts, in appropriate geometry, a convectively unstable electromagnetic wave having frequency

, where

, of the FEL type, where

. Megawatt level radiation at 1.7 mm, together with millimeter-harmonics and other microwave emission, is observed in a simple Fabry-Perot resonator system driven above oscillation threshold by a sufficiently large undulator field amplitude. Comparison of spectroscopic data with features of the theory permits identification of the cyclotron-undulator interaction.