A high‐power microwave propagating through a plasma‐filled waveguide gives rise to the generation of second and third harmonic waves because of ponderomotive force and energy‐dependent collisions, respectively. The generated harmonics possess a very different kind of intensity distribution than that of the normal modes. The efficiency of second harmonic generation is resonantly enhanced at some optimum size of the waveguide such that 2&bgr;1=&bgr;2, i.e.,n2&pgr;2/b2+(m2−4) &pgr;2/a2= 3&ohgr;p2/c2where &bgr;1and &bgr;2are the propagation constants of fundamental and second harmonic modes. This effect might account for the observed high yield of second harmonic generation in laser‐produced plasmas. The third harmonic power, however, shows a monotonic rise (tending towards a saturation value) with the width of the waveguide.