  We consider a \textit{mass-asymmetric} electron and hole bilayer. Electron
and hole Coulomb correlations and electron and hole quantum effects are treated
on first princles by path integral Monte Carlo methods. For a fixed layer
separation we vary the mass ratio $M$ of holes and electrons between 1 and 100
and analyze the structural changes in the system. While, for the chosen
density, the electrons are in a nearly homogeneous state, the hole arrangement
changes from homogeneous to localized, with increasing $M$ which is verified
for both, mesoscopic bilayers in a parabolic trap and for a macroscopic system.
