The effect of magnetic viscosity is investigated in two simple stability problems — Rayleigh-Taylor instability and Kelvin-Helmholtz instability of superposed fluids. In the Rayleigh-Taylor instability, perturbations perpendicular to the magnetic field are suppressed. In the Kelvin-Helmholtz instability, a magnetic field transverse to the direction of streaming velocities in general opposes the development of instability.