  Two-stream instability requires, essentially, two things to operate: a
relative flow between two fluids and some type of interaction between them. In
this letter we provide the first demonstration that this mechanism may be
active in a cosmological context. Building on a recently developed formalism
for cosmological models with two, interpenetrating fluids with a relative flow
between them, we show that two-stream instability may be triggered during the
transition from one fluid domination to the other. We also demonstrate that the
cosmological expansion eventually shuts down the instability by driving to zero
the relativeflow and the coupling between the two fluids.
