As there are a significant number of observed onset conditions for the post-sunset equatorial spread F that are not evidently associated with the required (5%) seed perturbation as assumed by the earlier simulation studies, an investigation is carried out with different seeding conditions for the evolution of plasma bubbles in the equatorial F region by means of a nonlinear numerical simulation model. The present investigation reveals that even an order of magnitude less perturbation (0.5%) than the earlier belief, grows into a bubble provided the nonlinear effects of other agencies such as the vertical wind and the electric field, in addition to gravity are taken into account. It is suggested that the above amplitude of the initial perturbation in plasma density can be generated through electrodynamical effects by the observed fluctuations in the electric fields. ¿ American Geophysical Union 1995