Linear theory of the drift-mirror instability in high-&bgr; plasma is reconsidered referring to basic principles for the two cases of a one- and a two-component ion plasma in presence of a cold electron background. In both cases the cold electrons serve to shortcut the parallel electric field component which imposes the condition of vanishing the field-aligned current. The corresponding low-frequency dispersion relation is derived in the fluid approximation as well as from kinetic theory including nonvanishing gradients in the density. The free energy of the unstable mode is taken from two sources: the pressure anisotropy and the spatial inhomogeneity of the plasma. The dispersion relation contains a correction which originates from the inclusion of the bending of the magnetic field that is caused by the reaction of the field to the total pressure force. It is shown that the mirror force substantially reduces the phase velocity which is in favor of instability since this requires phase velocities less than the drift speed. The direction of phase velocity becomes antiparallel to that of the pure density-gradient drift velocity. Even for a mirror-stable plasma an instability arises which is solely due to inhomogeneity. We analyze the transition to the classical mirror instability. Application to ring current (hot ring current ions plus cold plasmasphere ions) and magnetosheath (hot sheath ions only) conditions is presented and is in agreement with observational indication of the apparent stability of the pure mirror mode in the ring current. ¿ 2001 American Geophysical Union