  Near--infrared, optical and ultraviolet quasi--simultaneous observations of
11 BL Lacertae objects are reported. For all but one source the dereddened
spectral flux distribution in the $8\cdot10^{13}-2\cdot10^{15}$ Hz frequency
range can be described by a single power law f$_\nu \propto \nu^{-\alpha}$ with
average spectral index $<\alpha>$ = 0.88 $\pm$ 0.42 (standard deviation) plus,
where relevant, the contribution of the host galaxy. In most cases the non
simultaneous soft X--ray fluxes obtained by the {\it Einstein Observatory} lie
on or below the extrapolation of the power law. The results are compared with
the average spectral properties of other samples of BL Lacs studied separately
in the IR--optical and in the UV bands. The implications for existing models of
the objects are shortly discussed.
