  We have studied the transition from two to three dimensions in a low
temperature weakly interacting $^6$Li Fermi gas. Below a critical atom number,
$N_{2D}$, only the lowest transverse vibrational state of a highly anisotropic
oblate trapping potential is occupied and the gas is two-dimensional. Above
$N_{2D}$ the Fermi gas enters the quasi-2D regime where shell structure
associated with the filling of individual transverse oscillator states is
apparent. This dimensional crossover is demonstrated through measurements of
the cloud size and aspect ratio versus atom number.
