A Gaussian electromagnetic beam propagating in a plasma in the direction of the static magnetic field gives rise to (i) a ponderomotive force transverse to the magnetic field and (ii) nonuniform heating of electrons. These effects eventually modify the local magnetic field and thus produce a nonlinearity relevant to self‐focusing. The critical power for self‐focusing on a time scale shorter than the diffusion time (&tgr;D≳t) isvth/ctimes shorter than that on the longer time scale (t≳&tgr;D), wherevthis the thermal speed of electrons.