  Structural parameters (half-light radius $r_e$, mean effective surface
brightness $<\mu>_e$, and Sersic index n) are derived for a sample of galaxies
in the rich cluster AC 118 at z=0.31: so far the largest (N=93) sample of
galaxies at intermediate-redshift with structural parameters measured in the
near-infrared. The parameters are obtained in two optical wavebands (V and R)
and in the K-band. The distributions of $r_e$ at z=0.31 match those for the
Coma cluster both in the optical and in the NIR. The similarity of the two
distributions proves that the galaxies at the bright end of the luminosity
function did not significantly change their sizes since $z\sim0.3$ to the
present epoch. The ratio of the optical to the NIR half-light radius shows a
marked trend with the Sersic index n. In galaxies with $n\gtrsim4$ (typical
bright ellipticals) $r_{e,NIR}\sim 0.6 r_{e,opt}$, while the average ratio is
0.8 for galaxies with lower n (typical disk systems). Moreover, the NIR Sersic
index is systematically larger than in the optical for $n\lesssim4$. These
results, translated into optical and optical-NIR color gradients, imply that
the optical color gradients at $z\sim$0.3 are similar to those of nearby
galaxies. The optical-NIR color gradients are larger, ranging from -0.73
mag/dex for $n\lesssim4$ to -0.35 mag/dex for $n\gtrsim4$. We discuss these
results in terms of 'pure age' and 'pure metallicity' gradient models. The lack
of any major change in $r_{e,NIR}$ since $z\sim0.3$ suggests that merging
involving bright galaxies did not play a significant role in the last $\sim$4.4
Gyr.
