Sources such as atmospheric or buried explosions and shallow earthquakes producing strong vertical ground displacements are known to produce pressure waves that propagate at infrasonic speeds in the atmosphere. At ionospheric altitudes low frequency acoustic waves are coupled to ionospheric gravity waves and induce variations in the ionospheric electron density. Using Global Positioning System (GPS) data recorded by the permanent GPS network operating in southern California, we computed ionospheric electron content time series for several days preceding and following the January 17, 1994, MW=6.7 Northridge earthquake. We observe an anomalous signal beginning several minutes after the earthquake with time delays that increase with distance from the epicenter. The signal frequency and phase velocity are consistent with results from numerical models of atmospheric-ionospheric acoustic-gravity wave excited by seismic sources as well as previous electromagnetic sounding results. We believe that these perturbations are caused by the ionospheric response to the strong ground displacement associated with the Northridge earthquake. ¿ American Geophysical Union 1995