The dispersion equation for radial, axial and azimuthal instabilities of an isothermal disc with a three dimensional magnetic field is derived by the perturbation method. The effects of the different field components are discussed in detail. We find that the radial and azimuthal fields B r, B are mainly responsible for enhancing the instability of the magneto acoutic modes and non axisymmetric modes, and that pulsational oscillations are larger in the radial than in the Z direction. The model may be useful in explaining the...
