  The London penetration depth, $\lambda(T)$, was measured in polycrystalline
powders of the non-oxide perovskite superconductor $\mathrm{MgCNi_3}$ by using
a sensitive tunnel-diode resonator technique. The penetration depth exhibits
distinctly non s-wave BCS low-temperature behavior, instead showing quadratic
temperature dependence, suggestive of a nodal order parameter.
