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We report on the optical properties of a sample of 30 BL Lac host galaxies inthe redshift range $0.03<z<0.2$, as derived from HST observations. All galaxiesare fully resolved in the WFPC2 (F702W filter) images, allowing a quantitativeanalysis in two dimensions. Most and possibly all these galaxies havecharacteristics very similar to those of ``normal'' giant ellipticals. Theluminosity, ellipticity, isophote twisting and amount of disky or boxyisophotes are consistent with those found in non-active ellipticals and inradio galaxies. In all cases the BL Lac nucleus is well centered in the mainbody of its host galaxy, a result that argues strongly against the microlensinghypothesis for any significant fraction of the population. A search for faintsub-structures in the host galaxies has not revealed notable signatures oftidal distortions or sub-components (faint disks, bars, X features, etc.), andwith only one exception, there are no prominent dusty features in the centralregions. Instead, the BL Lac host galaxies are smooth and unperturbed,suggesting that strong external gravitational interactions are not important toongoing activity. A careful examination of the environment around the nucleus,however, shows a high incidence of close companion objects, whose natureremains unclear pending spectroscopic observations.Comment: 16 pages, 8 figures. Ap.J accepte

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