On August 20, 1996, balloon-borne X-ray detectors observed an intense X-ray event as part of a French balloon campaign near Kiruna, Sweden, at 1532 UT (1835 magnetic local time), on an L shell of 5.8. The energy spectrum of this event shows the presence of X rays with energies >1 MeV, which are best accounted for by atmospheric bremsstrahlung from monoenergetic ~1.7 MeV precipitating electrons. Ultraviolet images from the Polar satellite and energetic particle data from the Los Alamos geosynchronous satellites show the onset of a small magnetospheric substorm 24 min before the start of the relativistic electron precipitation event. Since the balloon was south of the auroral oval and there was no associated increase in relativistic electron flux at geosynchronous altitude, the event is interpreted as the result of selective precipitation of ambient relativistic electrons from the radiation belts. Pitch angle scattering caused by resonance with electromagnetic ion cyclotron mode waves is the most likely mechanism for selective precipitation of MeV electrons. A model is presented in which wave growth is driven by temperature anisotropies in the drifting substorm-injected proton population. The model predicts that this wave growth and resonance with ~1.7 MeV electrons will occur preferentially in regions of density >10 cm-3, such as inside the duskside plasmapause bulge or detached plasma regions. The model predictions are consistent with the location of the balloon, the observed energies, and the timing with respect to the substorm energetic particle injection. ¿ 2000 American Geophysical Union