Abstract The gravitational instabilities of a system consisting of two homogenous fluids in concentric cylindrical configuration (theta-pinch), in the presence of a uniform axial magnetic field, are investigated. The fluids are assumed to be incompressible, and viscous, with infinite conductivity. The dispersion relation is developed by two methods. In the first, an exact determination is made based upon the time development of an infinitesimal disturbance of the equilibrium conditions. In the...