  In this {\it Letter}, we make an attempt to estimate the masses of black
holes and their distributions in BL Lac objects. We find there is a bimodal
population of black hole masses in BL Lacs: the first has a lower-mass peak
centred at $\sim 7.0 \times 10^6\sunm$ composed of high frequency-peaked BL
Lacs (HBLs) and the second is a higher-mass ($>10^9\sunm$) population composed
of low frequency-peaked BL Lacs (LBLs). The low mass family of black holes in
HBLs does not satisfy the Magorrian et al. relation whereas the high mass
family in LBLs does this relation and is in agreement with Laor's limit. The
origination of the bimodal distribution of black hole mass remains open.
