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 ^ { -1 } corresponding to M _ { BH } of 5 \times 10 ^ { 7 } to 1 \times 10 ^ { 9 } M _ { \odot } .
The average ratio of M _ { BH } to the host galaxy mass is 1.4 \times 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 .