A giant undulation event observed at SANAE, Antarctica (L=4) is reported. During the event the geomagnetic activity was high, as was the case in previously reported observations of giant undulations. Data were recorded by all-sky and small field-of-view TV cameras, from 2346 UT on September 10, to 0007 UT on September 11 (MLT ~2200 h) 1987. The fine temporal and spatial resolution of the TV data made a detailed study of the undulation possible. The average wavelength was 170 km and the amplitude of the wave increased from 70 km to 140 km in 10 minutes. Wave propagation, measured at different positions on the undulation, was duskward with a phase speed of 540 to 650 m/s. An inner structure, propagating at a speed of 2.5 to 4.0 km/s, about 3 times as fast as the undulation form, was observed just poleward of the undulation boundary, indicating the presence of strong velocity shear. The linear theory of the velocity shear instability is examined as a possible generation mechanism on the basis of these observations. ¿ American Geophysical Union 1994