We analyze the hydrodynamic modes associated with carriers in a photoinjected plasma in a direct-gap polar semiconductor. In this paper we concentrate our attention on the carrier material motion. Resorting to a mechanostatistical approach to irreversible thermodynamics, the coupled equations of evolution for the charge density of electrons and holes are derived. They have the form of hyperbolic equations which imply collective damped wave propagation. The kinetic coefficients on which they depend are determined at this mechanostatistical microscopic level. The corresponding hydrodynamic modes are characterized: they consist of, besides the single quasiparticle (electrons and holes) excitations, two collective modes, namely, the acoustical and optical plasma waves. Their dispersion relations and lifetimes are obtained. These hydrodynamic modes are evidenced and further characterized through the calculation and analysis of the spectrum of Raman scattering of radiation by the carriers. The motion of the charge density is analyzed and we show how the damped wave regime goes over to the Fick-like diffusive regime in the limit of very large wavelengths and very low frequencies. © 1995 The American Physical Society.52750215029