Magnetospheric substorms start with strong temporal and spatial variations of a large number of magnetospheric parameters. We report on the occurrence of bidirectional field-aligned beams of energetic electrons (16 keV≤E≤80 keV) that were observed on the geosynchronous satellite GEOS 2. The electron beams appear about 4 min after the onset of the substorm expansion phase and last about 1.5 min. They are associated with strong spatial gradients of the ion intensity and fluctuations of the geoelectric and geomagnetic fields. The beams are interpreted as the result of an acceleration of electrons in the parallel electric field of kinetic Alfv¿n waves. It is assumed that these waves are generated by mode conversion from a surface wave that is generated by a magnetohydrodynamic instability in the transition region between dipolelike and taillike geomagnetic field lines. ¿ American Geophysical Union 1988