The correlation between black hole mass M(BH) and 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 derived values of sigma are in the range of 160 - 290 km/s corresponding to M(BH) of 5 x 10^7 to 1 x 10^9 Msun. 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 velocity dispersions and the derived values of 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.
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