  We consider the early universe at temperatures close to the fundamental scale
of gravity (M_D << M_Planck) in models with extra dimensions. At such
temperatures a small fraction of particles will experience transplanckian
collisions that may result in microscopic black holes (BHs). BHs colder than
the environment will gain mass, and as they grow their temperature drops
further. We study the dynamics of a system (a black hole gas) defined by
radiation at a given temperature coupled to a distribution of BHs of different
mass. Our analysis includes the production of BHs in photon-photon collisions,
BH evaporation, the absorption of radiation, collisions of two BHs to give a
larger one, and the effects of the expansion. We show that the system may
follow two different generic paths depending on the initial temperature of the
plasma.
