In order to study nonlinear interactions of water group ions with large amplitude whistler wave packets detected at the leading edge of steepended magnetosonic waves observed near comet Giacobini-Zinner, we carried out test particle simulations of water ion interactions with a model wave based on the G-Z data. As the model wave, we adopted a linearly polarized magnetosonic (MS) wave as the trailing portion of the wave, and circularly polarized whistler waves in the leading edge of the wave. Both the MS and whistler waves are a priori assumed to have large amplitudes. We found that some of the water ions are decelerated in the steepended portion of the magnetosonic wave to the resonance velocity with the whistler wave packets. Through resonance and related nonlinear interaction with the large amplitude whistler waves, the water ions become trapped by the whistler wave packet. An energy balance calculation demonstrates that the trapped ions do lose their kinetic energy during the trapped motion in the whistler wave packet. It is thus demonstrated that the nonlinear trapping motion in the wave structure leads to the effective energy transfer from the water group ions to the whisler wave packets in the leading edge of the steepended MHD waves. ¿ American Geophysical Union 1989