We report the first satellite observations of relativistic (>1 MeV) electron precipitation in microbursts with measured durations of less than 1 s. Microbursts of lower-energy electrons (10--100 keV) have been found to occur preferentially in the early daylight hours and to be closely associated with VLF chorus emissions. In contrast, the relativistic electron microbursts occurred more frequency near 2230 LT than 1030 LT, and no association was found with ELF/VLF chorus, consistent with the fact that resonant interactions with ~1-MeV electrons require significantly lower frequencies. The available data on these relativistic microbursts thus appear to indicate that many of the bursts may be due to wave-particle interactions not with whistler mode chorus but possibly with other waveforms. The locations of many of the relativistic microbursts are concentrated at the outer edge of the trapped radiation belt, where the gyroradii of the electrons are comparable to the curvature of the magnetic field lines and stable trapping may therefore not occur. The preferred location of the microbursts, which may be primarily spatial in character, implies the possible importance of irregularities in the magnetic field lines near the trapping boundary as the responsible mechanism. ¿ American Geophysical Union 1992