We investigate a subharmonic-resonant interaction between ions and magnetohydrodynamic waves propagating perpendicular to the background magnetic field, B0. This interaction occurs at frequencies which are subharmonics of the cyclotron frequency of ions (&OHgr;i/n, where n is an integer, 2,3,4,...). We have found that this interaction becomes quite effective if the wave magnetic field ΔB exceeds ~10% of B0. Further, when ΔB exceeds ~50% of B0, ions have stochastic orbits in the wave and are accelerated with quite high efficiency. This subharmonic resonant interaction can play quite significant roles in the ion heating/acceleration processes in space plasmas, where large amplitude waves (ΔB/B0>0.1) are not unusual. Possible observational features of the subharmonic resonance which we suggest are: (1) ''absorption lines'' in the broad background spectrum of compressible geomagnetic pulsations, and (2) enhancement of energetic ion fluxes relating to the large fluctuation of the magnetic field magnitude. ¿ American Geophysical Union 1989