For the first time, measurements from the Global Positioning System (GPS) worldwide network are employed to study the global ionospheric total electron content (TEC) changes during a magnetic storm (November 26, 1994). These measurements are obtained from more than 60 world-wide GPS stations which continuously receive dual-frequency signals. Based on the delays of the signals, we have generated high resolution global ionospheric maps (GIM) of TEC at 15 minute intervals. Using a differential method comparing storm time maps with quiet time maps, we find that significant TEC increases (the positive effect) are the major feature in the winter hemisphere during this storm (the maximum percent change relative to quiet times is about 150%). During this particular storm, there is almost no negative phase. A traveling ionospheric disturbance (TID) event is identified that propagates from the northern subauroral region to lower latitudes (down to about 30 ¿N) at a speed of ~460 m/s. This TID is coincident with significant increases in the TEC around the noon sector. We also find that another strong TEC enhancement occurs in the pre-dawn sector in the northern hemispheric subauroral latitudes, in the beginning of the storm main phase. This enhancement then spreads into almost the entire nightside. The nighttime TEC increase in the subauroral region is also noted in the southern hemisphere, but is less significant. These preliminary results indicate that the differential mapping method, which is based on GPS network measurements, appears to be a powerful tool for studying the global pattern and evolution process of the entire ionospheric perturbation. ¿ American Geophysical Union 1996