Time evolution of filamentation of an electromagnetic wave in the F region of the ionosphere has been studied numerically. It is seen that a plane electromagnetic beam of uniform power flux becomes unstable and gets filamented in the presence of transverse density fluctuations caused by a high power electromagnetic Gaussian beam. The computations are performed using Gaussian beams of 10, 30 and 50 μw/m2 power flux, to produce the nonuniform plasma fluctuations in the medium. The intensity of the plane wave is, however, fixed at 30 μw/m2. The filamentation of the plane wave develops in the entire region of the 100 km altitude range between 200 and 300 km used in the computations. The amplitude of the filaments increases with height and the growth is more rapid than an exponential variation. The plasma imitates the wave structure and the fluctuations in the electron temperature and density are field aligned. The relative density fluctuations associated with filaments lie in the range 0.1 to 1.5% and depend on the intensity of the incident Gaussian beam.