Average statistical characteristics of internal gravity waves (IGWs) with horizontal wavelengths longer than 60 km are obtained at the altitudes 65--80 km from the observations with the middle and upper atmosphere radar at Shigaraki, Japan (35 ¿N, 136 ¿E) in 1986--1995. The numbers of signals selected with narrow frequency band filters attributed to IGW having periods 0.1--6 hours vary between 870 and 2300 for different seasons and altitudes. Long-period IGWs have larger amplitudes and larger vertical wavelengths than short-period IGWs. For waves with periods 0.1--6 hours the mean values of the horizontal and vertical wavelengths are 100--300 km and 6--10 km, respectively. IGW amplitudes and momentum fluxes have maxima in winter and summer and minima in spring and autumn. Distributions of the azimuths of IGW horizontal wave vectors and momentum fluxes are nearly isotropic with maxima in zonal directions in summer. The westward momentum flux is larger than the eastward flux in winter and is smaller than that in summer. The differences between eastward and westward wave momentum fluxes are larger for long-period IGWs. The net zonal IGW momentum flux is mainly directed opposite to the mean wind in the mesosphere. ¿ 2000 American Geophysical Union