Measurements on the beam-plasma interaction occurring when an energetic ion beam is injected into a plasma are presented. Excitation of ion cyclotron harmonic waves propagating azimuthally across the applied magnetic field is observed. The excitation is attributed to the small velocity component of the ion beam directed perpendicular to the magnetic field, independent of the parallel velocity component. The experimental results are compared with the linear theory of electrostatic instabilities in a Maxwellian plasma traversed by a monoenergetic beam with d. c. energy components parallel and perpendicular to the magnetic field.