Rocketsonde observations at Kwajalein (8.7¿N, 167.7¿E) were used for investigation of horizontal wind disturbances induced by inertial instability. Two characteristic cases were found in the 1978--1979 northern winter during which the limb infrared monitor of the stratosphere (LIMS) made observations. In both the cases the eastward component and the northward component of horizontal winds are negatively correlated in the vertical direction near the stratopause level, which is thought to be due to inertial instability. The wind disturbances have a wavelength of about 10 km, and their maximum amplitudes are estimated as over 10 ms-1. Examinations of the LIMS temperatures support a possibility that the wind disturbances are induced by inertial instability. It was revealed that temperature disturbances characteristic of inertial instability, called pancake structures, appear in the same height and time as the wind disturbances do. The relationship between the rocketsonde wind disturbances and the LIMS pancake structures is 90¿ out of phase, as expected by the theory of inertial instability. In addition, it was confirmed that the two inertial instability events follow enhancements of the midlatitude planetary wave. This is consistent with a mechanism of the inertially unstable disturbances in the equatorial region inferred in some observational and numerical studies. Analyses of rocketsonde data were extended to the entire observation period at Kwajalein, which suggests that the occurrence of inertially unstable disturbances is biased toward the winter season. However, at some other stations in the tropics, wind disturbances which may be induced by inertial instability do not appear so frequently as they do at Kwajalein. This seasonality and regionality of inertially unstable disturbances would be due to the seasonality and regionality of planetary wave breakings at midlatitude.