A fully integrated microstrip oscillator is described. The active element is an InP transferred-electron device operating in its fundamental mode of oscillation. For measurement purposes, the oscillator structure is mounted in a test assembly and fitted with a waveguide to microstrip transition. Output powers of 16 mW have been observed at the waveguide output port at frequencies in the range 80 GHz to 83 GHz. Transition and circuit losses are approximately 3 dB indicating a device output power of 32 mW.