  Replacing U for Ca in semiconducting CaB$_6$ at the few at.% level induces
metallic behaviour and Kondo-type phenomena at low temperatures, a rather
unusual feature for U impurities in metallic hosts. For
Ca$_{0.992}$U$_{0.008}$B$_6$, the resistance minimum occurs at $T$ = 17 K. The
subsequent characteristic logarithmic increase of the resistivity with
decreasing temperature merges into the expected $T^2$ dependence below 0.8 K.
Data of the low-temperature specific heat and the magnetization are analyzed by
employing a simple resonance-level model. Analogous measurements on LaB$_6$
with a small amount of U revealed no traces of Kondo behavior, above 0.4 K.
