The first successful electric field measurement in the ionosphere using the time-of-flight technique with a lithium ion beam was carried out on a S-520 sounding rocket launched from Kagoshima Space Center, Japan on January 15, 1987. The purpose of this experiment was to prove the validity of the time-of-flight technique when it is applied to the measurement of the electric field in the ionosphere. A time-coded ion beam was ejected from the rocket in the direction perpendicular to the Earth's magnetic field. The beam returned to the rocket twice per rocket spin when the initial beam direction was nearly perpendicular to the electric field. The electric field and the magnetic field were derived from the travel time of these return lithium ions. The accuracy of the electric field determination was ¿0.3 mV/m. The direction of the electric field was obtained from the direction of the returning ion beam after about one ion gyration. The main constituent of the measured electric field was a V¿B field due to the rocket motion across the geomagnetic field. The ambient field was less than 1 mV/m. The magnetic field was measured with an accuracy of ¿2.7 nT in this experiment. ¿ American Geophysical Union 1989