This paper presents an investigation of nonlinear third‐harmonic generation in a weakly collisional magnetoplasma due to simultaneous propagation of both the right and left circularly polarized modes, having a Gaussian intensity distribution; self‐focusing has been taken into account. At moderate powers, the self‐focusing is seen to enhance the harmonic output by two orders of magnitude; at high powers, propagation occurs in an almost uniform waveguide devoid of plasma, and the harmonic output is, consequently, decreased. In the vicinity (&ohgr;c/&ohgr;=0.7) of the electron cyclotron resonance, the harmonic output of the extraordinary mode is enhanced by an order of magnitude; the present theory is not applicable at resonance.