  We have measured the central stellar velocity dispersion in the host galaxies
of 11 BL Lac objects with redshifts z <= 0.125. The range of velocity
dispersions, 170-370 km/s, is similar to that of nearby radio galaxies. Using
the correlation between stellar velocity dispersion and black hole mass defined
for nearby galaxies, we derive estimates of the black hole masses in the range
10^7.9 - 10^9.2 solar masses. We do not find any significant difference between
the black hole masses in high-frequency-peaked and low-frequency-peaked BL Lac
objects. Combining the velocity dispersions with previously measured host
galaxy structural parameters, we find that the host galaxies lie on the
fundamental plane of elliptical galaxies. This supports the conclusions of
imaging studies that the majority of BL Lac hosts are normal giant ellipticals.
