Bremsstrahlung X ray (>21 keV) mappings and direct electron (>68 keV) measurements from the low-altitude polar-orbiting satellite P78-1 were performed at critical times and locations near the onset of a magnetospheric substorm on July 3, 1979 at ~2149 UT. The bremsstrahlung X ray intensities emitted from the atmosphere over a wide range of longitudes and L shells were negligible just before the substorm onset, then rose to a maximum within about a minute or less, and remained at an enhanced level for the next 9 minutes. The onset time for the total intensities of X rays observed from the P78-1 satellite was nearly the same as for the X ray flux increase at one of the SBARMO-79 balloons which were at fortuitous positions during the event and within the field of view of the satellite X ray detectors. The simultaneous X ray measurements from the SBARMO-79 balloons and the P78-1 satellite indicate that the electron precipitation started at L=5-5.5 without a significant precipitation from higher L shells. In addition, the satellite data revealed a relatively sharp longitude decrease in electron precipitation at positions east of ~1.5 hours.