Host galaxies and cluster environment of BL Lac objects at z>0.5

First results from an extensive study of the properties of the host galaxies and cluster environment of BL Lac objects at z > 0.5 are presented. The main aim of this study is to confirm the apparent evolution previously claimed. A host galaxy was detected in all BL Lac objects at z < 0.7. At higher redshifts our search was not successful. An exception might be PKS 0426-380 (z > 1.03). Here we detected a galaxy, which might be either along the line of sight or the host galaxy of the BL Lac itself. If the latter is correct, the upper limit to the redshift is wrong, A clear overdensity of galaxies have been found for two BL Lac objects, a slight overdensity for two others. In the remaining sources no overdensity of galaxies could be detected. Spectroscopy of the environment of PKS 0537-441 (z = 0.894) confirmed the physical association of at least 4 galaxies with the BL Lac. A previously discussed lensing scenario for this source can be ruled out with high confidence.