An increasing number of high-resolution spacecraft observations identify magnetic fluctuations in various space plasma environments as mirror mode structures. In a collisionless plasma the excitation of mirror modes has to be analyzed in a kinetic approach, where details of the velocity space distributions are of considerable significance. Nonthermal, high-energy tail distributions, commonly detected in planetary magnetospheres and the interplanetary medium, are known to be accurately modeled by the family of kappa distributions. It is demonstrated that the conventional fluid instability criterion is inadequate since suprathermal particle populations provide a strong source for the generation of mirror wave mode activity even in the low-&bgr; limit and small temperature anisotropy. The specific structure of velocity space distributions at high energies dominates as regulating mechanism for the mirror instability threshold. ¿ 2000 American Geophysical Union