A spectacular gravity wave event was observed from Haleakala on a four-color all-sky imager, as well as in OH Meinel-band intensity and temperature, on October 10, 1993, during the Airborne Lidar and Observations of Hawaiian Airglow (ALOHA-93) campaign <Taylor et al. 1995>. The event consisted of a sharp frontal passage in airglow brightness propagating at 76 m/s across the field of view and accompanied by small-scale waves behind the front. The purpose of this paper is to propose an explanation for the frontal passage in terms of an internal mesospheric undular bore. Although there have been a number of observations of internal bores in the troposphere and in the ocean, we believe this is the first reported observation of such a bore in the mesosphere. A simple two-layer model, constructed by analogy with the theory of bores in river channels, explains the observations quantitatively. Several predictions are advanced which could serve to test the model. ¿ 1998 American Geophysical Union