Observations from a thermal ion probe on board a Defense Meteorological Satellite Program satellite have been used to study the longitudinal distribution of equatorial spread F occurrence in the topside ionosphere. The data were taken in the postsunrise local time sector during the September 1977 magnetic storm. In agreement with previous studies of the longitudinal variability of evening sector spread F, a higher frequency of occurrence was found in the American than in the Asian sector. No example of spread F was observed in the Atlantic-African sector. The results were interpreted using a quasi-three-dimensional model of the collisional Rayleigh-Taylor instability. It is shown that for the same initial ionospheric conditions and the same initial depletion at the magnetic equator, large plasma bubbles are more apt to reach the topside at longitudes of the American sector than at longitudes of the Asian sector.