  The correlation between black hole mass M(BH) and the central stellar
velocity dispersion sigma in nearby elliptical galaxies affords a novel way to
determine M(BH) in active galaxies. We report on measurements of sigma from
optical spectra of 7 BL Lac host galaxies. The range of sigma (160 - 290 km/s)
corresponds to estimated M(BH) of 5 x 10^7 - 9 x 10^8 Mo. The average ratio of
M(BH) to the host galaxy mass is 1.4 x 10^-3, consistent with that estimated in
other active and inactive galaxies. The range of sigma and M(BH) of the BL Lacs
are similar to those obtained for low redshift radio galaxies, in good
agreement with the predictions of the unified models for radio-loud active
galaxies.
