  PKS 2155-304 is one of the brightest extragalactic source in the X-ray and
EUV bands, and is a prototype for the BL Lac class of objects. In this paper we
investigate the large-scale environment of this source using new multi-object
as well as long-slit spectroscopy, together with archival spectra and optical
images. We find clear evidence of a modest overdensity of galaxies at
z=0.11610, consistent with previous determinations of the BL Lac redshift. The
galaxy group has a radial velocity dispersion of 250km/s and a virial radius of
0.22Mpc, yielding a role-of-thumb estimate of the virial mass of
M(vir)~1.5x10$^{13}$Msun, i.e., one order of magnitude less than what observed
in other similar objects. This result hints toward a relatively wide diversity
in the environmental properties of BL Lac objects.
