  We investigate the propagation of electromagnetic waves in resistive pair
plasmas using a onefluid theory derived from the relativistic two-fluid
equations. When the resistivity normalized by the electron/positron inertia
variable exceeds a critical value, the dispersion relation for electromagnetic
waves shows that the group velocity is larger than the light speed in vacuum.
However, in such a case, it also is found that the plasma parameter is less
than unity: that is, the electron-positron pair medium no longer can be treated
as plasma. Thus the simple two-fluid approximation is invalid. This confirms
that superluminal propagation of electromagnetic wave is forbidden in a plasma
-- a conclusion consistent with the relativistic principle of causality. As an
alternative, we propose a new set of equations for ``causal relativistic
magnetohydrodynamics", which both have non-zero resistivity and yet are
consistent with the causality principle.
