Bursty radio emissions are often observed from the polar magnetospheres of the Earth, Jupiter, Saturn, and Uranus in addition to the smooth radio emissions commonly detected. We show that in plasma regimes in which the electron plasma frequency is less than the electron cyclotron frequency, anisotropic electron beams or gyrating electron beams can excite directly broadband electromagnetic radiation. The largest growth is for right-hand X-mode radiation with frequencies above the electron cyclotron frequency. This instability can produce bursty, broadband emission, consistent with some of the properties of the radiation observed from the magnetized planets. ¿American Geophysical Union 1990