A purely kinetic instability of the dust acoustic mode in inhomogeneous plasmas is discussed. In the presence of a magnetic field, electrons and ions may be magnetized while at the same time dust grains may remain un-magnetized. Although the dynamics of the light species is strongly affected by the magnetic field, the dust acoustic mode may still propagate in practically any direction. The inhomogeneity implies a source of free energy for an instability that develops through the diamagnetic drift effects of the magnetized species.