  We present a theoretical model of the "isostructural" \gamma-\alpha phase
transition in Ce which is based on quadrupolar interactions due to coupled
charge density fluctuations of 4f electrons and of conduction electrons.
Conduction electrons are treated in tight-binding approximation. The
\gamma-\alpha transition is described as an orientational ordering of
quadrupolar electronic densities in a Pa3 structure. The quadrupolar order of
the conduction electron densities is complementary to the quadrupolar order of
4f electron densities. The inclusion of conduction electrons leads to an
increase of the lattice contraction at the \gamma-\alpha transition in
comparison to the sole effect of 4f electrons. We calculate the Bragg
scattering law and suggest synchrotron radiation experiments in order to check
the Pa3 structure. The theory is capable of accounting for transitions to
phases of non-cubic symmetry, but it is not sufficient to describe the magnetic
phenomena which we ascribe to the Kondo mechanism. We also present a
microscopic derivation of multipolar interactions and discuss the crystal field
of \gamma-Ce.
