The parametric interaction of waves with particle cyclotron motion has been studied. A wave with a compressional magnetic field component modifies the cyclotron motion of the particles, which enables them to resonate with other waves which have perpendicular electric fields. Resonance occurs when the frequency difference of the two wave modes (compressional and perpendicular) matches the cyclotron frequency. This so-called parametric cyclotron resonance mechanism can provide a means to exchange energy between particles and low-frequency waves. We discuss the specific example of energy exchange between an Alfv¿n wave and ion Larmor motion. ¿ American Geophysical Union 1993