  We present some preliminary results related to a project aimed at studying
the evolution of the galaxy population in rich environments by means of the
Color-Magnitude relation and of the Fundamental Plane. We derive the NIR and
optical structural parameters for a sample of galaxies in the cluster AC118 at
z=0.31. We prove that reliable structural parameters of galaxies at z$\sim$0.3
can be still derived from ground--based observations. The NIR effective radii,
measured for the first time at this redshift, turn out to be significantly
smaller than those derived from the optical data, providing new insight into
the evolution of colour gradients in galaxies.
