A hexapole magnetic field has been superimposed on the fast magnetic mirror compression field of the Scylla III &thgr; pinch in an attempt to stabilize an observed plasma instability. The instability is of the flute type with a superimposed rotation. The effect of the hexapole field on the plasma stability was studied with streak camera photography and through observation of the neutron emissions. Under certain conditions the hexapole ``stabilizing'' field has an observable effect on the instability. Its main effect is to symmetrize the rotation about the axis of the compression coil and to produce a delay in the onset of the instability. There is at most a small effect tending to symmetrize the time distribution of neutron emission and to increase the neutron yield.An ionized deuterium plasma is produced by a fast, low energy capacitor bank coupled directly to the single‐turn compression coil of the Scylla III device. The ``afterglow'' of this preionization discharge, uniformly permeated by a magnetic bias fieldB0, forms the initial condition for the application of the main magnetic compression field. The effect of the main compression field on the preionized plasma depends strongly on theB0which exists in the plasma.