Hot flow anomalies (HFAs) are localized plasma structures observed in the solar wind and magnetosheath near the Earth's quasi-parallel bow shock. We present 1-D hybrid computer simulations illustrating a formation mechanism for HFAs in which the single, hot, ion population results from a spatial separation of two counterstreaming ion beams. The higher-density, cooler regions are dominated by the background (solar wind) ions, and the lower-density, hotter, internal regions are dominated by the beam ions. The spatial separation of the beam and background is caused by the deflection of the ions in large amplitude magnetic fields which are generated by ion/ion streaming instabilities. ¿American Geophysical Union 1990