The longitude structure of ionospheric total electron content (TEC) at low latitudes has been evaluated using the NASA/Center Nationale d'Etudes Spatiales TOPEX/Poseidon satellite. The TEC data set is given by the ionospheric range correction, which is computed from TOPEX dual-frequency altimeter measurements. The satellite's orbit allows analysis of vertically measured TEC values at approximately 30¿ intervals of longitude across the world at local time differences of only 6--12 min. Patterns of longitudinal dependence of the equatorial anomaly were observed during the equinoxes, summers, and winters of 1993, 1994, and 1995. TOPEX observations reveal occurrence of relative maximum anomaly TEC values in the Indian/Asian longitude sector. This dominance in TEC is seen most consistently in the Asian Southern Hemisphere. Also, a relative decrease in anomaly TEC values is evident in the western American region, which is observed primarily during equinox and winter. This configuration of the equatorial anomaly TEC is observed on a day-to-day basis at particular periods of local time. Global theoretical ionospheric model results are presented in an attempt to reproduce the distinctive longitude structure. Variability in E¿B vertical drift velocity within specific longitude sectors is shown to be a primary factor in the longitude dependence of equatorial anomaly TEC. ¿ 1999 American Geophysical Union