Natural space plasmas generally exhibit a pronounced high-energy tail distribution that can best be modeled by a generalized Lorentzian (kappa) distribution. We employ the recently introduced modified plasma dispersion function [Summers and Thorne, 1991> to obtain the dispersion relation for field-aligned electromagnetic waves in such a plasma, and use this to study the instability properties of R mode and L mode waves in the solar wind and in planetary magnetospheres. We demonstrate for a wide range of plasma parameters that the growth of R mode waves in the solar wind can be significantly enhanced by the presence of a pronounced high-energy tail; previous studies based on a Maxwellian distribution could therefore be seriously in error. The corresponding enhancement in the growth rate of L mode waves in planetary magnetospheres is less dramatic, but the kappa distribution tends to produce significant wave amplification over a broader range of frequency than a Maxwellian distribution with comparable bulk properties. At frequencies comparable to the ion gyrofrequency wave growth is primarily caused by cyclotron resonance with ions. Hot anisotropic electrons can nevertheless influence the instability as a result of changes in the wave phase velocity. This modulating effect is most important for a Maxwellian plasma and becomes less significant as the spectral index of the kappa distribution is reduced. ¿ American Geophysical Union 1993