  We present time-resolved observations of Rayleigh--Taylor-instability (RTI)
evolution at the interface between an unmagnetized plasma jet colliding with a
stagnated, magnetized plasma. The observed instability growth time ($\sim
10$~$\mu$s) is consistent with the estimated linear RTI growth rate calculated
using experimentally inferred values of density ($\sim 10^{14}$~cm$^{-3}$) and
deceleration ($\sim 10^9$~m/s$^2$). The observed mode wavelength ($\gtrsim
1$~cm) nearly doubles within a linear growth time. Theoretical estimates of
magnetic and viscous stabilization and idealized magnetohydrodynamic
simulations including a physical viscosity model both suggest that the observed
instability evolution is subject to magnetic and/or viscous effects.
