Ionospheric irregularities and their zonal apparent drift were studied using Global Positioning System (GPS) measurements at Cachoeira Paulista (22.41 ¿S, 45.00 ¿W, -26¿ dip angle) in Brazil during November 6--19, 1998. Radio scintillations at the GPS L1 frequency (1.575 GHz) were monitored using four GPS receivers spaced geomagnetically east-west and north-south. Total electron content (TEC) was measured through the ionospheric advance of the GPS L1 and L2 (1.227 GHz) phases. Strong amplitude scintillations coincided with TEC fluctuations associated with spread F bubbles elongated along the magnetic field. Movement of the Fresnel-scale (400 m) ionospheric irregularity layers caused the scintillation to drift, and their zonal apparent drift velocities were measured using a cross-correlation technique. Our measurements show that the apparent eastward velocity varies from 200 m/s to 150 m/s at 2000 LT, and then it decreases to 100--50 m/s at midnight. On a magnetically disturbed day, reversal of the zonal apparent drift was observed just after midnight, and the apparent westward velocities observed at early in the morning showed large variations with location in the sky. From the receivers spaced in the geomagnetic north-south direction we measured near-zero time shifts, from which we conclude that the correlation length of several-hundred-meter-scale irregularities is much larger than 70-m separation between the north and south receivers. ¿ 2000 American Geophysical Union