The average relative flow velocity between protons and alpha particles in collisionless plasmas can excite several distinct alpha/proton instabilities. Here linear theory and two-dimensional hybrid simulations in a homogeneous plasma model are used to study one such mode, the alpha/proton magnetosonic instability, using plasma parameters characteristic of the high-speed solar wind observed by the Ulysses space-craft at high heliospheric latitudes. Wave-particle scattering by enhanced fluctuations from this mode reduces the alpha/proton relative speed and heats the alphas more strongly than the protons. Post-saturation results for the alpha/proton flow in the simulations are approximately bounded by the linear theory threshold condition for onset of the magnetosonic instability, which is also a statistical upper bound on the alpha/proton relative speed at high &bgr;p for some of the Ulysses observations. Thus it is likely that this instability is a constraining agent for the alpha/proton relative speed in some domains of the solar wind. ¿ 2000 American Geophysical Union