[en] The interaction of a rotating ion beam with a plasma in an external magnetic field is investigated. The ion beams have an energy component perpendicular to the magnetic field which is large compared with the energy of the background plasma ions. It is shown that the interacton leads to the excitation of ion cyclotron harmonic waves with a growth rate which can be as large as ω/sub c//sub i//2. It is also shown, using a theoretical argument, that the growth rate for the excitation of ion cyclotron waves by a purely longitudinal beam is comparatively small