  We report on measurements of the stellar velocity dispersion (sigma) from the
optical spectra of the host galaxies of four BL Lac objects. Together with our
earlier results on seven BL Lac objects (Falomo et al. 2002), and with the
previously derived photometrical and structural properties, these data are used
to construct the Fundamental Plane (FP) of the BL Lac hosts. We find that the
BL Lacs follow the same FP as low redshift radio galaxies and inactive luminous
ellipticals, in agreement with similar results presented by Barth et al.
(2003). This indicates that the photometrical, structural and kinematical
properties of the host galaxies of BL Lacs are indistinguishable from those of
inactive massive ellipticals. Using the correlation between black hole mass
(M_BH) and sigma in nearby elliptical galaxies, we derive the masses of the
central black hole in BL Lacs. These masses, in the range of 6 x 10^7 to 9 x
10^8 M_o, are consistent with the values derived from the bulge luminosity and
appear to be linearly correlated with the mass of the galaxies (M_BH} ~0.001 x
M_bulge).
