DC electric fields were measured by the Electric Field Detector-Beam experiment (EFD-B) onboard the sounding rocket S-310-26 above the E-region ionosphere during the Sporadic-E Experiment over Kyushu (SEEK) campaign. The sounding rocket was launched into the ionosphere when a strong quasi-periodic (QP) echo structure appeared. The EFD-B measured both the electric field and the magnetic field from X+169s until X+232s (altitude of 167 km to 173 km). Accuracy of the measurements were 0.19 mV/m for the electric field and 1.9 nT for the magnetic field. The magnetic field measurements were in very good agreement with a model field (IGRF 95). The electric fields, which were projections of the ionospheric electric fields to the altitude of the observation point along the magnetic field line, reached to 11.5 mV/m at maximum. The observed electric field is much larger than expected, and is associated with the 4 km periodic structure of the electric field which may be related to the atmospheric gravity wave activity in the E region. ¿ 1998 American Geophysical Union