Both electron and ion cyclotron waves are important as an effective source of auxiliary heating in plasmas. When high-power radiation is applied to a plasma (as much as a few MW), it produces nonlinear effects that affect the propagation and absorption of beams of waves. Experimental observations of cyclotron oscillations (TFR Group, 197 5) show that they play an important role in the behaviour of laboratory plasmas. Cyclotron waves also occur widely in the Earth's magnetosphere (Russel et al., 1970; Gurnett, 1974; Bergmann, 1984). In particular, intensive emission of electromagnetic waves in the electron cyclotron frequency range has been recorded in the auroral region; this is the Earth's kilometre wavelength radiation (Gurnett, 1974; Alexander and Kaiser, 1977; Cole and Pokhotelov, 1980). In a number of cases, cyclotron waves are observed to have large amplitudes and to be localized in space (Mozer et al., 1977), possibly as a consequence of nonlinearity.