We have performed numerical simulations of nonlinear evolution of the Rayleigh-Taylor instability in equatorial spread F (ESF) under different conditions. Our main purpose is to explain the generation of multiple plumes on the west wall of a plasma upwelling and the relationship between gravity waves and large scale ESF irregularities. We have studied ESF structures resulting from one-dimensional or two-dimensional initial density perturbations and initiated by gravity waves. It is found that although plasma bubbles may be produced in the all situations, the production of the plasma bubbles initiated by gravity waves takes a time much shorter than that resulting from two-dimensional initial density perturbations. This is in agreement with a simple analytic nonlinear theory. We put our emphasis on the ESF structures growing from two different scale perturbations. The Rayleigh-Taylor instability initiated by gravity waves produces a steep gradient on the west wall, which provides a favorable condition for excitation of smaller-scale secondary instabilities. When both a seed gravity wave and a smaller-scale initial density perturbation are used, it is observed that the large-scale gravity wave determines the outer scale of ESF irregularities by seeding the Rayleigh-Taylor instability and the smaller-scale perturbation results in multiple plumes preferentially located on the west wall. The production, rise, and bifurcation of plasma bubbles are reproduced in the simulations. ¿ American Geophysical Union 1996