  Instabilities in magnetic fields wound up by differential rotation as
reviewed in Spruit (1999) are discussed with some detail and new developments
added. In stellar models which include magnetic torques, the differential
rotation tends to accumulate in the gradients in composition. In view of this,
instability in a $\mu$-gradient is studied in more detail here, resulting in
the detection of a second instability. Its relevance for angular momentum
transport is uncertain, however, since it requires high horizontal field
gradients and would not operate near the pole. Finally, the possibility is
discussed that magnetic instability in a $\mu$-gradient will lead to {\em layer
formation}: the gradients breaking up into small steps of uniform composition
and rotation rate. This would enhance the angular momentum transport across
inhomogeneous zones, and decrease the rotation rates of the end products of
stellar evolution. A recent {\tt astro-ph} submission by Dennisenkov and
Pinsonneault proposing a modification of the instability conditions is shown to
contain a mathematical error.
