One-dimensional hybrid computer simulations are used to study electromagnetic instabilities driven by hot, anisotropic, counterstreaming proton components similar to those observed by ISEE-1 and -2 in the plasma sheet boundary layer of Earth's magnetotail. These simulations are used to study two types of growing modes: the ion cyclotron anisotropy instability driven by the temperature anisotropy of individual ion components, and two different ion/ion instabilities driven by the relative streaming of two ion components. As in previous such simulations, the ion cyclotron anisotropy instability exhibits a very low saturation amplitude and weak ion scattering. Simulations of the current-driven kink-like instability using more general particle-particle codes yield similarly weak fluctuation amplitudes and very small ion scattering. If the relative streaming speed of the hot proton components is increased to greater than 2vA, the proton/proton nonresonant instability can grow to saturation at larger amplitudes and produce strong proton scattering which leads to significant heating of the two hot proton distributions. Furthermore, if a relatively large density of cool ionospheric oxygen ions is introduced in the simulations, the ion/ion right-hand resonant instability becomes the dominant mode, and pitch angle scatters the hot proton components to produce distributions which resemble those observed from ISEE. Thus electromagnetic ion/ion instabilities are a possible mechanism by which the hot streaming proton components of the plasma sheet boundary layer become the single very hot isotropic proton component of the central plasma sheet. ¿ American Geophysical Union 1990