  We report on detailed low temperature measurements of the magnetization, the
specific heat and the electrical resistivity on high quality CeRuSn single
crystals. The compound orders antiferromagnetically at $T_{\rm N} = 2.8$ K with
the Ce$^{3+}$ ions locked within the $a-c$ plane of the monoclinic structure.
Magnetization shows that below $T_{\rm N}$ CeRuSn undergoes a metamagnetic
transition when applying a magnetic field of 1.5 and 0.8 T along the $a$ and
$c$--axis, respectively. This transition manifests in a tremendous negative
jump of $\sim 25$% in the magnetoresistance. The value of the saturated
magnetization along the easy magnetization direction ($c$--axis) and the
magnetic entropy above $T_{\rm N}$ derived from specific heat data correspond
to the scenario where only one third of the Ce ions in the compound being
trivalent and carrying a stable Ce$^{3+}$ magnetic moment, whereas the other
two thirds of the Ce ions are in a nonmagnetic tetravalent and/or mixed valence
state. This is consistent with the low temperature CeRuSn crystal structure
i.\,e.\,, a superstructure consisting of three unit cells of the CeCoAl-type
piled up along the $c$--axis, and in which the Ce$^{3+}$ ions are characterized
by large distances from the Ru ligands while the Ce-Ru distances of the other
Ce ions are much shorter causing a strong 4{\it f}-ligand hybridization and
hence leading to tetravalent and/or mixed valence Ce ions.
