The wave‐particle interactions of a hot ion beam streaming along a magnetic fieldBare studied. A second‐order theory of electromagnetic instabilities in a homogeneous, collisionless plasma at propagation parallel toBis used. The two instabilities most likely to be driven by a hot beam are the right‐hand and left‐hand resonant ion beam instabilities. If the conditions necessary for the validity of the theory are met, the two modes are found to reinforce one another. That is, each mode acts to produce a beam anisotropy which reduces its own growth rate but enhances the growth rate of the other. Thus this theory predicts that whenever sufficiently hot or “diffuse” ions are found at a collisionless shock and the plasma is sufficiently homogeneous that significant wave growth is possible, ion beam instabilities will act to produce a mixture of both right‐ and left‐hand polarized