We report on the first experimental determination of the magnetic field-aligned electric field (E∥) of an Alfv¿n/ion cyclotron (A/I) wave. The wave was observed propagating in the geostationary regions of the Earth's magnetosphere with a frequency of 1/3 the proton cyclotron frequency. Electron oscillations parallel to (B) are used in the determination of E∥. E∥ is found to be a factor of ~18 greater than anticipated from cold plasma theory (i.e., electron inertial effects). The inclusion of electron pressure and pressure anisotropy brings reasonable agreement between the observations and linear wave theory. ¿ American Geophysical Union 1993