Microwave‐radiation bursts at &lgr;∼1 cm are observed when a rotating relativistic electron beam interacts with a plasma. The power level exceeds 1 MW, and the radiation pulse lasts as long as the electron beam pulse for beams up to 1 &mgr;sec long. The results are consistent with a model of coherent curvature radiation from electrons bunched due to a two‐stream instability. Other mechanisms are ruled out by observations of harmonic splittings and the power spectrum.