A commonly observed feature of the field-aligned flow of ions on closed geomagnetic field lines is that the pitch angle distributions are considerably broadened. Such pitch angle distributions facilitate in trapping ions and thus contribute to the process of plasmaspheric refilling. Since counterstreaming ion beams are a common feature of the early stage refilling as shown by models, we have investigated the role of ion-beam-driven instabilities in broadening the pitch angle distribution of the ion beams. The study is performed using a 2.5-dimensional particle-in-cell code. It is shown that for sufficiently fast ion beams as expected in the outer region of the plasmasphere, the ion cyclotron modes, which appear at frequencies less than the ion-cyclotron frequency &OHgr;i in the rest frame of the nondrifting electrons, are the dominant unstable wave modes. In the early stage of the instability a nearly purely growing mode dominates, which efficiently mediates in transferring a part of the parallel drift energy into the perpendicular energy, significantly broadening the ion pitch angle distribution. The transfer abruptly saturates when Jn(k⊥&rgr;if)→0, where Jn( ) is the Bessel function of an appropriate order n, k⊥ is the perpendicular wave number of the dominant mode, and &rgr;if is the maximum value of the Larmor radius of the beam ions in anomalous cyclotron resonance with the wave. This behavior of the dominant mode in conjunction with the conservation of ion energy allows a method for including such wave-particle interaction effects in mesoscale models for the plasmaspheric refilling. The perpendicular acceleration of ions by this instability reduces the parallel drift only by a relatively small fraction, leaving the counterstreaming intact. However, the perpendicular acceleration produces a pitch angle distribution which is inducive for the trapping of the counterstreaming ions in the plasmaspheric flux tubes. ¿ 1999 American Geophysical Union