The nonlinear evolution of the local collisional Rayleigh-Taylor instability in downward-moving equatorial F layers has been studied in a coordinated theoretical and experimental program dealing with actual conditions of bottomside spread F. For ambient bottomside electron density gradient scale lengths L = 8 and 25 km, we find large percentage depletions and inverse power law spatial power spectra over the intermediate wavelength range &lgr; = 25 m-1 km. In addition we outline a nonlinear theory of the collisional Rayleigh-Taylor instability applicable to an upward- or downward-moving equatorial F region ionosphere. The agreement between the experimental and theoretical results of this study lends further support to the belief that the collisional Rayleigh-Taylor instability is responsible for large-scale size irregularities that occur under conditions of equatorial spread F.