In order to clarify the origin of the so-called quasi periodic echoes (QPE) that have been often detected by radar observations in the presence of sporadic-E (Es) layers in the nighttime midlatitude ionosphere, two sounding rockets were launched during the SEEK (Sporadic-E Experiment over Kyushu) campaign. Each rocket carried a swept-frequency impedance probe to measure the E-region electron-density (Ne) profile. Using the four Ne profiles obtained during the two rocket flights together with a neutral-wind profile obtained from a trimethyl aluminum (TMA) chemical release experiment on one of the rockets and QPE obtained with a ground-based radar, we consider the role of wind shear in the formation of the observed Es layers, and the question of whether QPE are associated with Es layers that are modulated in altitude. The Ne profiles of Es structures that were obtained in the presence of QPE were characterized by the highly concentrated thin layers. The formation of such a thin layer by a neutral-wind shear process was confirmed in comparison with the TMA measurements. The peak Ne values of the Es layers ranged from 2.2 to 9.3¿104 el/cm3 near 100-km altitude. These primary Es layers were accompanied by significant secondary structures that were located about 12 to 20 km above the main Es layers and had peak Ne that ranged from 5.2¿103 to 1.3¿104 el/cm3. The average altitude profiles of QPE approximately covered the range where the Es-layer peaks appeared. Our principal finding is that the observed Es structures tended to resemble horizontally stratified layers rather than structures with deep altitude modulation like previous QPE model, although the rocket measurements were separated from those by radar by 90 to 145 km. ¿ 1998 American Geophysical Union