  We study the effects of plasma viscosity on the dynamics of primordial
magnetic fields by simulating magnetohydrodynamics in the early universe by
appropriate non-linear cascade models. We find numerically that even in the
presence of large kinetic viscosity, magnetic energy is transferred to large
length scales. There are indications, however, that the inverse cascade stops
at a given time which depends on the magnitude of viscosity. For realistic
viscosities we do not find equipartition between magnetic and kinetic energies.
