  We study the classical dynamics in a generic first-order quantum phase
transition between the U(5) and SU(3) limits of the interacting boson model.
The dynamics is chaotic, of H\'enon-Heiles type, in the spherical phase and is
regular, yet sensitive to local degeneracies, in the deformed phase. Both types
of dynamics persist in the coexistence region resulting in a divided phase
space.
