OH airglow (about 87 km altitude) was observed with a CCD imager at Shigaraki (34.9¿N, 136.1¿E) in August 1996, during the SEEK campaign period. From nine night observations, the characteristics of the gravity wave modulation of the OH airglow layer were extracted and analyzed. The distributions of the horizontal wavelength, observed wave period and horizontal phase speed were 10--30 km, 5--30 min and 20--60 m/s, respectively. The horizontal propagation direction was mainly north-eastward. This preferential direction is similar to previous MU radar observations of mesospheric gravity waves (68--80 km) in summer months, but different from the propagation direction of gravity waves expected from FAI theories and observations in the E-region. ¿ 1998 American Geophysical Union