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[en] Using the earlier expressions for nonlinear dielectric constant (epsilon = epsilon₀ + PHI (EE*) of isotropic and magnetoactive plasmas, we have investigated the self focusing of gaussian electromagnetic beams in short (t much less than tau/sub epsilon/), and long (t greater than tau/sub epsilon/) time scales; tau/sub epsilon/ is the energy relaxation time of electrons with heavy particles and epsilon₀ = epsilon (EE* = 0). The wave equation has been solved in the paraxial ray and WKB approximations by expanding epsilon as epsilon = epsilon/sub a/ (EE* vertical bar r = 0) + γr². Unlike previous analyses no restriction has been put on the relative magnitudes of PHI and epsilon₀. Consequently, the treatment is applicable beyond critical electron density. In a magnetoplasma, the self focusing of extra-ordinary mode has been studied and the treatment is applicable for all values of plasma frequency. Saturating effects of nonlinearity have been discussed in detail. Special attention to self focusing of whistlers and interpretation of Stenzel's experimental results has been given. We have also studied the filamentation of a plane uniform whistler and the growth rate of filamentation instability is seen to be a saturating function of the intensity of the beam

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