  We have measured the magneto-optical Kerr rotation of ferromagnetic
$Eu_{1-x}Ca_{x}B_{6}$ with x=0.2 and 0.4, as well as of $YbB_{6}$ serving as
the non-magnetic reference material. As previously for $EuB_{6}$, we could
identify a feature at 1 $eV$ in the Kerr response which is related with
electronic transitions involving the localized 4f electron states. The absence
of this feature in the data for $YbB_{6}$ confirms the relevance of the
partially occupied 4f states in shaping the magneto-optical features of
$Eu$-based hexaborides. Disorder by $Ca$-doping broadens the itinerant charge
carrier contribution to the magneto-optical spectra.
