  We present high-resolution imaging in the rest-frame optical of the weak
radio source LBDS53W091. Previous optical spectroscopy has shown that this
object has an evolved stellar population of age >3Gyr at z=1.55, determined
from the amplitude of rest-frame UV spectral breaks. We have obtained deep
Hubble Space Telescope imaging over 10 orbits with NICMOS camera 2, using the
F160W H-band filter (1.6microns) which is a good approximation to the
rest-frame R-band. Our observations reveal a radial light profile which is well
fit by a de Vaucouleurs' r^{1/4} law, with a scale length of r_e=0.3"
(3kpc/h_{50} for Omega_M=0.3 and Omega_{Lambda}=0). The elliptical morphology
of the radio galaxy indicates a dynamically-evolved old system, consistent with
the spectroscopic results. Some surrounding objects lie on the color:magnitude
relation for a cluster at z=1.55, and are likely to be associated. The group of
galaxies are somewhat more luminous than the fundamental plane of ellipticals
at z=0 subject to (1+z)^4 surface brightness dimming, but are consistent with
estimates of the fundamental plane at high redshift subject to passive
luminosity evolution from a formation epoch of z>3.
