  We demonstrate, both at the Newtonian and (general) relativistic levels, the
existence of a generalization of Kelvin's circulation theorem (for pure fluids)
which is applicable to perfect magnetohydrodynamics. The argument is based on
the least action principle for magnetohydrodynamic flow. Examples of the new
conservation law are furnished. The new theorem should be helpful in
identifying new kinds of vortex phenomena distinct from magnetic ropes or fluid
vortices.
