  We study the instability of a moving spinor Bose-Einstein condensate when the
speed of flow reaches the critical velocity. This we identify on the basis of
Landau's criterion, i.e. the velocity above which some elementary excitation
energy becomes negative. We show that the first-to-become unstable excitations
are spin-carrying quasiparticles. We also discuss the temperature dependence of
the critical velocity in a more advanced mean-field approximation.
