We report a new theory to explain the long-observed frequency-doubling phenomenon detected in the compressional component of high-m ultralow-frequency signals seen in the Earth's magnetosphere. We show that the effect is a nonlinear response directly associated with resonant particles. The phase relationship observed between the linear signal and the nonlinear frequency-doubled signal is predicted by the theory. Our explanation and the fit with observation mean that the occurrence of frequency doubling provides critical evidence of the fact that ring current particles drive high-m waves. ¿ 1997 American Geophysical Union