Heating of heavy ions has been observed in the equatorial magnetosphere in GEOS 1 and 2 and ATS 6 data due to ion cyclotron waves generated by anisotropic hot ring current ions. A one-dimensional hybrid-Darwin code has been developed to study ion heating in the ring current. The mechanism for the heating of thermal hydrogen and helium by ion cyclotron waves has been studied previously in particle simulations of a bi-Maxwellian plasma. Here, a strong instability and heating of thermal ions is investigated in a plasma with a loss cone distribution of hot ions. The linear growth rate calculation and particle simulations are conducted for cases with different loss cones and relative ion densities. The linear instability of the waves, the quasi-linear heating of cold ions and dependence on the thermal H+/He+ density ratio are analyzed, as well as nonlinear parallel heating of thermal ions. Effects of thermal oxygen and hot oxygen are also studied. ¿ American Geophysical Union 1990