Varian is carrying out the development of high-power, cw gyrotrons at frequencies ranging from 85-140 GHz. An experimental tube employing a TE22 ,2 , 1 -mode cavity and utilizing an output coupling concept where the collection of the spent electron beam is separated from the outgoing microwave power, has been designed and constructed. In initial tests on the tube, output powers of 450-500 kW have been obtained for pulse durations out to 2.5 seconds, in the desired TE22,2,1 mode at a frequency of 110 GHz. Output powers up to 1 MW have been achieved for short pulses (1 ms). Future tests on this and subsequent experimental tubes will be aimed at achieving 1 MW power levels under long-pulse and ultimately, cw conditions.