  We present high-resolution imaging and low-resolution spectroscopy of the BL
Lac object PKS 0537-441 (z = 0.893) and its environment. The observations were
designed to clarify, whether the properties of PKS 0537-441 are affected by
gravitational microlensing, or whether PKS 0537-441 and its environment act as
a lensing system itself. Our observations show that neither case seems to be
likely. We did not find a galaxy along the line-of-sight to the BL Lac as
claimed previously, our spectroscopy shows that none of the four closest
companion galaxies is at high redshift. Two of the four nearby companion
galaxies to PKS 0537-441 are within 200 km/s of the systemic velocity of the BL
Lac (z = 0.892 and 0.895, respectively). The third companion galaxy is at
higher redshift (z = 0.947). The fourth companion galaxy shows evidence of Mg
II absorption redwards of its systemic velocity and is perhaps a mini low
ionization BAL QSO at z = 0.885. If the latter can be confirmed, PKS 0537-441
is the first BL Lacertae object being a member of a binary Quasar. We also
detected extended [O II] emission in the off-nuclear spectrum of PKS 05371-441,
which is most likely due to photoionization from the active nucleus.
Alternatively, the extended [O II] emission is due to jet-cloud interaction
with the counterjet of PKS 0537-441. Our clustering analysis indicates that PKS
0537-441 is located in a cluster environment as rich as Abell type 0-1. This is
supported by the detection of four galaxies in the field with similar redshifts
as the BL Lac (Delta z < 0.002). We found serendipitously even more galaxies at
somewhat higher redshifts (z = 0.9-1). Thus, PKS0537-441 might be located in
front of a galaxy cluster at somewhat higher redshift or even be part of a
large-scale structure with an extension towards the BL Lac.
