  A general formalism for calculating the bulk viscosity of strange quark
matter is developed. Contrary to the common belief that the non-leptonic
processes alone give the dominant contribution to the bulk viscosity, the
inclusion of the Urca processes is shown to play an important role at
intermediate densities when the characteristic r-mode oscillation frequencies
are not too high. The interplay of non-leptonic and Urca processes is analyzed
in detail
