This paper develops the linear gradient drift instability theory of middle latitude ES -layer irregularities, taking into account the appropriate field geometry and allowing for the fact that, in addition to a predominant perpendicular component, the propagation vector of an irregularity can have a small but significant component parallel to the magnetic field as well. The theory is used to calculate the growth rates and phase and group velocities of the unstable waves as a function of the off-perpendicular propagation angle. Group propagation effects are also considered.