This paper describes results from simultaneous measurements of mid-latitude irregularities by a sounding rocket and a Doppler radar. The experiment was conducted on August 26, 1996 as part of the SEEK (Sporadic-E Experiment over Kyushu) campaign in Japan. Electric field observed with a double-probe sensor on the rocket showed very large fluctuations of ¿10 mV/m where irregularity echoes were quite intense. Doppler velocities observed with the radar agreed well with the electric fields. We estimated growth rates of the gradient-drift instability, which showed positive values around the major sporadic-E layer. It was found that, within intense irregularity region with quasi-periodic structures, there were large polarization electric fields that was likely to be associated with spatial structure of the sporadic-E layer. ¿ 1998 American Geophysical Union