  X-ray powder diffraction, magnetic susceptibility $\chi(T)$, and electrical
resistivity $\rho(T)$ measurements were made on single crystals of the filled
skutterudite series Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$. One end of the series
($x = 0$) is a heavy fermion superconductor with a superconducting critical
temperature $T_{c} = 1.85$ K, while the other end ($x = 1$) is a conventional
superconductor with $T_{c} \approx 1$ K. The lattice constant $a$ decreases
approximately linearly with increasing Ru concentration $x$. As Ru (Os) is
substituted for Os (Ru), $T_{c}$ decreases nearly linearly with substituent
concentration and exhibits a minimum with a value of $T_{c} = 0.75$ K at $x =
0.6$, suggesting that the two types of superconductivity compete with one
another. Crystalline electric field (CEF) effects in $\chi_\mathrm{dc}(T)$ and
$\rho(T)$ due to the splitting of the Pr$^{3+}$ nine-fold degenerate Hund's
rule $J = 4$ multiplet are observed throughout the series, with the splitting
between the ground state and the first excited state increasing monotonically
as $x$ increases. The fits to the $\chi_\mathrm{dc}(T)$ and $\rho(T)$ data are
consistent with a $\Gamma_{3}$ doublet ground state for all values of x,
although reasonable fits can be obtained for a $\Gamma_{1}$ ground state for
$x$ values near the end member compounds ($x = 0$ or $x = 1$).
