This paper studies the ionospheric response to the major geomagnetic storm of October 18--19, 1995, using the NCAR TIE-GCM simulations and the global ionospheric maps (GIM) of total electron content (TEC) observations from the worldwide network of Global Positioning System (GPS) receivers. The TIE-GCM results show a good agreement with the GPS-GIM in terms of simulating storm-time TEC disturbances over the polar regions. The model indicates that the increase of electron density in the high-latitude E and lower F regions below 200 km is directly related to the magnetospheric energy input through auroral precipitation to the ionosphere, while the decrease of TEC is mainly due to the increase in O2 and N2 densities in the upper F region above 200 km. During the recovery phase, both the TIE-GCM and GPS-GIM reveal a distinct hemispheric asymmetry in the TEC integrated above |50¿| magnetic latitude, with a 20% decrease in the southern (summer-like) hemisphere and a 30% increase in the northern (winter-like) hemisphere. ¿ 1998 American Geophysical Union