Helium ash alpha-particles at 100keV in magnetically confi ned fusion plasmas may have the same Larmor radius, as well as cyclotron frequency, as the energetic beam-injected deuterons that heat the plasma. While the velocity-space distribution of the helium ash is monotonically decreasing, that of the energetic deuterons is a delta-function in the edge plasma. Here we identify, by means of fi rst principles particle-in-cell computations, a new physical process by which Larmor radius matching enables collective gyroresonant energy transfer between these two co-located minority energetic ion populations, embedded in majority thermal plasma. This newly identi ed phenomenon rests on similar underlying physics to widely observed ion cyclotron emission from suprathermal minority ion populations.