  We study the bulk viscosity of neutron star matter including $\Lambda$
hyperons in the presence of quantizing magnetic fields. Relaxation time and
bulk viscosity due to both the non-leptonic weak process involving $\Lambda$
hyperons and direct Urca processes are calculated here. In the presence of a
strong magnetic field of $10^{17}$ G, the hyperon bulk viscosity coefficient is
reduced whereas bulk viscosity coefficients due to direct Urca processes are
enhanced compared with their field free cases when many Landau levels are
populated by protons, electrons and muons.
