  The symmetry of the superconducting order parameter in single-crystalline
PuCoGa$_5$ ($T_{\rm c} = 18.5$ K) is investigated via zero- and transverse-
field muon spin relaxation ($\mu$SR) measurements, probing the possible
existence of orbital and/or spin moments (time reversal-symmetry violation TRV)
associated with the superconducting phase and the in-plane magnetic-field
penetration depth $\lambda(T)$ in the mixed state, respectively. We find no
evidence for TRV, and show that the superfluid density, or alternatively,
$\Delta\lambda(T) = \lambda(T) - \lambda(0)$, are $\propto T$ for $T/T_{\rm c}
\leq 0.5$. Taken together these measurements are consistent with an even-parity
(pseudo-spin singlet), d-wave pairing state.
