We report on direct imaging and spectroscopy of the field around the BL Lac object PKS 2155 - 304. An analysis of the richness of galaxies in the field shows that the source is located in a poor cluster of galaxies. Spectra of three galaxies indicate that they are at redshift z = 0.116, and the spectrum of the nebulosity surrounding the BL Lac object, yielding z = 0.116, shows that the BL Lac object belongs to the cluster. The galaxy environment of BL Lac objects, together with those of F-R I radio sources, the proposed parent objects in the beaming hypothesis, is briefly discussed.
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Falomo et al. (1993) studied this question.