A kinetic description of magnetized plasma flow with mass loading is developed and leads to a nonlinear wave equation for small scale dispersive magnetosonic plasma motions superimposed on large scale motions due to mass loading. The mass loading can drive quasi-stationary, finite-amplitude magnetosonic type waves in a supersonic plasma flow, even though small amplitude linear disturbances are damped, but only when the magnetosonic Mach number drops to 2 during the slowing down the of the mass loaded plasma flow. Nonlinear motions with constant amplitude could be permanent features of a mass loaded plasma flow, as in the neighborhood of comets and planets with extended atmospheres but negligible magnetic fields. The relation between the wave amplitude and the local magnetosonic Mach number is computed and can be compared with in situ measurements in cometary plasma environments. ¿American Geophysical Union 1990