  We investigate the Jeans instability of a galactic disk embedded in a
dynamically responsive dark halo. It is shown that the disk-halo system becomes
nominally Jeans unstable. On small scales the instability is suppressed, if the
Toomre stability index Q_T is higher than a certain threshold, but on large
scales the Jeans instability sets invariably in. However, using a simple
self-consistent disk-halo model it is demonstrated that this occurs on scales
which are much larger than the system so that this is indeed only a nominal
effect. From a practical point of view the Jeans instability of galactic disks
is not affected by a live dark halo.
