The dispersion relation is derived for parametric instabilities caused by an intense ion sound wave which couples high- and low-frequency electrostatic modes of a non-isothermal unmagnetized homogeneous plasma. The phase velocity of the pump wave is between the ion and the electron thermal speed. The dispersion relation contains filamentational, modulational, and stimulated scattering instabilities. Growth rates as a function of the pump and plasma characteristics are given. For a given amplitude of the pump, |ϕ0|, the largest growth rate is found in the case of filamentation instability. The particular physical situation studied in this paper is related only loosely to any actual R. F. heating experiment. However, it allows some new basic parametric processes occurring during R. F. heating below and around the ion gyrofrequency to be clarified. Finally, the relevance of this and other more generalparametric instabilities, from the point of view of plasma heating and confinement, is briefly discussed.