  The stability of large-scale magnetic fields in rotating stars is explored,
using 3D numerical hydrodynamics to follow the evolution of an initial poloidal
field. It is found that the field is subject to an instability, located
initially on the magnetic equator, whereby the gas is displaced in a direction
parallel to the magnetic axis. If the magnetic axis is parallel to the rotation
axis, the rotation does not affect the initial linear growth of the
instability, but does restrict the growth of the instability outside of the
equatorial zone. The magnetic energy decays on a timescale which is a function
of the Alfv\'en crossing time and the rotation speed, but short compared to any
evolutionary timescale. No evidence is found for a possible stable end state to
evolve from an initial axisymmetric poloidal field. The field of an oblique
rotator is similarly unstable, in both cases regardless of the rotation speed.
