  We report measurements of the magnetic penetration depth $\lambda$ in single
crystals of PrRu$_{4}$Sb$_{12}$ down to 0.1 K. Both $\lambda$ and superfluid
density $\rho_{s}$ exhibit an exponential behavior for $T$ $<$ 0.5$T_{c}$, with
parameters $\Delta$(0)/\textit{k}$_{B}$\textit{T}$_{c}$ = 1.9 and $\lambda(0)$
= 2900 \AA. The value of $\Delta$(0) is consistent with the specific-heat jump
value of $\Delta C/\gamma T_{c}$ = 1.87 measured elsewhere, while the value of
$\lambda(0)$ is consistent with the measured value of the electronic
heat-capacity coefficient $\gamma$. Our data are consistent with
PrRu$_{4}$Sb$_{12}$ being a moderate-coupling, fully-gapped superconductor. We
suggest experiments to study how the nature of the superconducting state
evolves with increasing Ru substitution for Os.
