The operation of a 92 GHz quasi-optical gyrotron having a resonator formed by a spherical mirror and a diffraction grating placed in -1 order Littrow mount is presented. A power of 150 kW with a Gaussian output pattern was measured. The Gaussian content in the output was 98% with less than 1% of depolarization. By optimizing the magnetic field al fixed frequency a maximum efficiency of 15% was reached. (C) 1996 American Institute of Physics.