  We study the X-ray spectra of 85 BL Lacertae objects using the hardness
ratios as given in the WGA catalogue of {\it ROSAT} sources. This sample
includes all WGA BL Lacs with high-quality data and comprises about 50 per cent
of presently known BL Lacs. We find that BL Lacs have energy power-law spectral
indices between 0 and 3 with a mean value $\alpha_{\rm x}\sim 1.4$. Significant
differences, however, are present between high-energy cutoff BL Lacs (HBLs),
normally selected in the X-ray band, and low-energy cutoff BL Lacs (LBLs),
generally found in radio surveys. HBLs have steeper X-ray spectral slopes
($\alpha_{\rm x}\sim 1.5$) well correlated with $\alpha_{\rm ox}$, and
anti-correlated with the X-ray-to-radio flux ratio and cutoff frequency, with
convex overall broad-band spectra. LBLs, on the other hand, have flatter X-ray
spectra ($\alpha_{\rm x}\sim 1.1$) and concave optical-X-ray continuum. We
interpret these results in terms of different mechanisms being responsible for
the X-ray emission in the two classes, namely synchrotron and inverse Compton
for HBLs and LBLs respectively. The observed differences are consistent with
the hypothesis that HBLs and LBLs are powered by essentially the same
non-thermal engines differing mainly in their synchrotron cutoff energy.
