  I calculate the linear stability of a stratified low collisionality plasma in
the presence of a weak magnetic field. Heat is assumed to flow only along
magnetic field lines. In the absence of a heat flux in the background plasma,
Balbus (2000) demonstrated that plasmas in which the temperature increases in
the direction of gravity are buoyantly unstable to convective-like motions (the
``magnetothermal instability''). I show that in the presence of a background
heat flux, an analogous instability is present when the temperature decreases
in the direction of gravity. The instability is driven by the background heat
flux and the fastest growing mode has a growth time of order the local
dynamical time. Thus, independent of the sign of the temperature gradient,
weakly magnetized low collisionality plasmas are unstable on a dynamical time
to magnetically-mediated buoyancy instabilities. The instability described in
this paper is predicted to be present in clusters of galaxies at radii from ~
0.1-100 kpc, where the observed temperature increases outwards. Possible
consequences for the origin of cluster magnetic fields, ``cooling flows,'' and
the thermodynamics of the intercluster medium are briefly discussed.
