  The impact of superthermal electrons on dispersion properties of isotropic
plasmas and on the modulational instability of a monochromatic Langmuir wave is
studied for the case when the power-law tail of the electron distribution
function extends to relativistic velocities and contains most of the plasma
kinetic energy. Such an energetic tail of electrons is shown to increase the
thermal correction to the Langmuir wave frequency, which is equivalent to the
increase of the effective electron temperature in the fluid approach, and has
almost no impact on the dispersion of ion-acoustic waves, in which the role of
temperature is played by the thermal spread of low-energy core electrons. It is
also found that the spectrum of modulational instability in the non-maxwellian
plasma narrows significantly, as compared to the equilibrium case, without
change of the maximum growth rate and the corresponding wavenumber.
