An investigation was carried out by means of a nonlinear numerical simulation of equatorial spread F (ESF) to understand the characteristics of the evolutionary pattern of plasma bubbles with different background topside plasma density distributions above the same bottomside distribution in the equatorial F region at the postsunset times. The investigation revealed that the upward drift of the plasma bubble significantly depends on fractional depletion which is controlled by the background topside electron density distribution. This in turn modifies the zonal and vertical extents of the plasma bubble. It is shown that the observed differences in the zonal widths and vertical extents of plasma bubbles over Kwajalein and Jicamarca regions are due to the differences in the plasma density distributions in the topside ionosphere over those regions. ¿ American Geophysical Union 1995